fix(stm32mp1): remove boolean check on PLAT_TBBR_IMG_DEF

This flag just needs to be defined, and does not need to have a boolean
value. Remove it from the assert_booleans check.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I4e4c9ae1e5003ca2cf7c0c0e31d1561d032937c8
1 file changed