KVM: add tracing & ebpf support in kernel

This allows me to use rkt to test ebpf-kprobes without impacting the
host kernel.

The KVM flavor currently includes the kernel version 4.8.6rkt-v1, which
is enough for my current needs.

tracefs is normally available in /sys/kernel/debug/tracing/ and is
necessary for kprobe support.

I could test in the following way:
```
sudo ./build-rkt-1.21.0+git/target/bin/rkt \
  run --interactive \
  --insecure-options=image,all-run \
  --dns=8.8.8.8 \
  --stage1-path=./build-rkt-1.21.0+git/target/bin/stage1-kvm.aci \
  --volume=ebpf,kind=host,source=$GOPATH/src/github.com/kinvolk/gobpf-elf-loader/ \
  docker://debian \
  --mount=volume=ebpf,target=/ebpf
```

And inside rkt:
```
mount -t tmpfs tmpfs /tmp # workaround rkt#1917
mount -t debugfs debugfs /sys/kernel/debug/
cd /ebpf
./gobpf-elf-loader ./ebpf.o
```

This patch increases the size of stage1-kvm.aci by 5MB (from 43MB to
48MB). Is it acceptable?

stage1/usr_from_kvm/kernel/cutdown-config

@@ -217,7 +217,7 @@ CONFIG_EPOLL=y
 CONFIG_SIGNALFD=y
 CONFIG_TIMERFD=y
 CONFIG_EVENTFD=y
-# CONFIG_BPF_SYSCALL is not set
+CONFIG_BPF_SYSCALL=y
 CONFIG_SHMEM=y
 CONFIG_AIO=y
 CONFIG_ADVISE_SYSCALLS=y
@@ -237,16 +237,21 @@ CONFIG_SLAB=y
 # CONFIG_SLUB is not set
 # CONFIG_SLAB_FREELIST_RANDOM is not set
 # CONFIG_SYSTEM_DATA_VERIFICATION is not set
-# CONFIG_PROFILING is not set
+CONFIG_PROFILING=y
+CONFIG_TRACEPOINTS=y
+CONFIG_OPROFILE=y
+# CONFIG_OPROFILE_EVENT_MULTIPLEX is not set
 CONFIG_HAVE_OPROFILE=y
 CONFIG_OPROFILE_NMI_TIMER=y
-# CONFIG_KPROBES is not set
+CONFIG_KPROBES=y
 CONFIG_JUMP_LABEL=y
 # CONFIG_STATIC_KEYS_SELFTEST is not set
+CONFIG_OPTPROBES=y
 # CONFIG_UPROBES is not set
 # CONFIG_HAVE_64BIT_ALIGNED_ACCESS is not set
 CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y
 CONFIG_ARCH_USE_BUILTIN_BSWAP=y
+CONFIG_KRETPROBES=y
 CONFIG_HAVE_IOREMAP_PROT=y
 CONFIG_HAVE_KPROBES=y
 CONFIG_HAVE_KRETPROBES=y
@@ -980,7 +985,7 @@ CONFIG_NET_CLS_U32=m
 # CONFIG_NET_CLS_RSVP6 is not set
 CONFIG_NET_CLS_FLOW=m
 CONFIG_NET_CLS_CGROUP=y
-# CONFIG_NET_CLS_BPF is not set
+CONFIG_NET_CLS_BPF=y
 # CONFIG_NET_CLS_FLOWER is not set
 # CONFIG_NET_CLS_MATCHALL is not set
 CONFIG_NET_EMATCH=y
@@ -1015,13 +1020,15 @@ CONFIG_SOCK_CGROUP_DATA=y
 CONFIG_CGROUP_NET_CLASSID=y
 CONFIG_NET_RX_BUSY_POLL=y
 CONFIG_BQL=y
-# CONFIG_BPF_JIT is not set
+CONFIG_BPF_JIT=y
 CONFIG_NET_FLOW_LIMIT=y
 
 #
 # Network testing
 #
 # CONFIG_NET_PKTGEN is not set
+# CONFIG_NET_TCPPROBE is not set
+# CONFIG_NET_DROP_MONITOR is not set
 # CONFIG_HAMRADIO is not set
 # CONFIG_CAN is not set
 # CONFIG_IRDA is not set
@@ -1980,6 +1987,7 @@ CONFIG_DEBUG_BUGVERBOSE=y
 # CONFIG_TORTURE_TEST is not set
 CONFIG_RCU_CPU_STALL_TIMEOUT=60
 CONFIG_USER_STACKTRACE_SUPPORT=y
+CONFIG_NOP_TRACER=y
 CONFIG_HAVE_FUNCTION_TRACER=y
 CONFIG_HAVE_FUNCTION_GRAPH_TRACER=y
 CONFIG_HAVE_FUNCTION_GRAPH_FP_TEST=y
@@ -1989,8 +1997,35 @@ CONFIG_HAVE_FTRACE_MCOUNT_RECORD=y
 CONFIG_HAVE_SYSCALL_TRACEPOINTS=y
 CONFIG_HAVE_FENTRY=y
 CONFIG_HAVE_C_RECORDMCOUNT=y
+CONFIG_TRACE_CLOCK=y
+CONFIG_RING_BUFFER=y
+CONFIG_EVENT_TRACING=y
+CONFIG_CONTEXT_SWITCH_TRACER=y
+CONFIG_RING_BUFFER_ALLOW_SWAP=y
+CONFIG_TRACING=y
 CONFIG_TRACING_SUPPORT=y
-# CONFIG_FTRACE is not set
+CONFIG_FTRACE=y
+# CONFIG_FUNCTION_TRACER is not set
+# CONFIG_IRQSOFF_TRACER is not set
+# CONFIG_SCHED_TRACER is not set
+# CONFIG_ENABLE_DEFAULT_TRACERS is not set
+# CONFIG_FTRACE_SYSCALLS is not set
+# CONFIG_TRACER_SNAPSHOT is not set
+CONFIG_BRANCH_PROFILE_NONE=y
+# CONFIG_PROFILE_ANNOTATED_BRANCHES is not set
+# CONFIG_PROFILE_ALL_BRANCHES is not set
+# CONFIG_STACK_TRACER is not set
+# CONFIG_BLK_DEV_IO_TRACE is not set
+CONFIG_KPROBE_EVENT=y
+# CONFIG_UPROBE_EVENT is not set
+CONFIG_BPF_EVENTS=y
+CONFIG_PROBE_EVENTS=y
+# CONFIG_MMIOTRACE is not set
+# CONFIG_HIST_TRIGGERS is not set
+# CONFIG_TRACEPOINT_BENCHMARK is not set
+# CONFIG_RING_BUFFER_BENCHMARK is not set
+# CONFIG_RING_BUFFER_STARTUP_TEST is not set
+# CONFIG_TRACE_ENUM_MAP_FILE is not set
 
 #
 # Runtime Testing
@@ -2239,7 +2274,7 @@ CONFIG_SYSTEM_TRUSTED_KEYS=""
 # CONFIG_SECONDARY_TRUSTED_KEYRING is not set
 CONFIG_HAVE_KVM=y
 # CONFIG_VIRTUALIZATION is not set
-# CONFIG_BINARY_PRINTF is not set
+CONFIG_BINARY_PRINTF=y
 
 #
 # Library routines