xtensa build defines Kconfigs with lowercase names

[zephyrbot]

Reported by Andrew Boie:

The Xtensa build uses CONFIG_$(XTENSA_CORE), which in many cases has lowercase letters. Fix this so that the configs follow naming convention, we probably need a per-SOC string Kconfig to tell the build what core build to fetch out of the SDK.

(Imported from Jira ZEP-1711)

[zephyrbot]

by Robert Beatty:

Moving to {{P3}}

As determined by Bug Triage, this is not a high-priority item and not fully featured.