Build: Simplify TFM_PLATFORM to support platform name only

Current TF-M build requires TFM_PLATFORM to be set as the relative
path to platform/ext/targets. If vendor platform folder structure
is changed, TFM_PLATFORM shall be adjusted accordingly, which may
break the backward combability of build command.

This patch will simplify the TFM_PLATFORM to support platform name
only. For example, -DTFM_PLATFORM=arm/msp2/an521 and
-DTFM_PLATFORM=an521 shall be both accepted.

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: I3b03f6dfa88cc1ee8059f7802f717e86676c377d
2 files changed