Building and flashing guide improvements #1884

Job	Run time
read_targets_from_file / read_targets_from_file	4s
basic_build (cf2)	3m 22s
basic_build (bolt)	3m 28s
basic_build (tag)	3m 17s
basic_build (flapper)	3m 21s
basic_build (cf21bl)	3m 22s
kbuild-targets (allyesconfig)	3m 9s
apps (examples/app_hello_world)	2m 40s
kbuild-targets (allnoconfig)	3m 11s
features (bosch.conf)	3m 16s
apps (examples/app_hello_world-cpp)	2m 46s
kbuild-targets (randconfig)	3m 14s
features (loco_tdma.conf)	3m 15s
apps (examples/app_hello_file_tree)	2m 42s
features (loco_tdoa2.conf)	3m 17s
apps (examples/app_peer_to_peer)	2m 43s
features (loco_tdoa3.conf)	3m 16s
features (loco_tdoa3_all.conf)	3m 16s
apps (examples/app_p2p_DTR)	2m 51s
features (bigquad.conf)	3m 24s
apps (examples/app_stm_gap8_cpx)	2m 56s
features (app_api.conf)	3m 24s
apps (examples/demos/app_push_demo)	2m 49s
apps (examples/demos/swarm_demo)	2m 47s
apps (examples/demos/app_wall_following_demo)	2m 42s
	1h 14m 32s

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