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
diff --git a/plat/st/stm32mp1/platform.mk b/plat/st/stm32mp1/platform.mk
index 1d93983..75c2383 100644
--- a/plat/st/stm32mp1/platform.mk
+++ b/plat/st/stm32mp1/platform.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2015-2023, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2015-2023, Arm Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -158,7 +158,6 @@
 	$(sort \
 		PKA_USE_BRAINPOOL_P256T1 \
 		PKA_USE_NIST_P256 \
-		PLAT_TBBR_IMG_DEF \
 		STM32MP_CRYPTO_ROM_LIB \
 		STM32MP_DDR_32BIT_INTERFACE \
 		STM32MP_DDR_DUAL_AXI_PORT \