tf_config(mbedtls): test out MBEDTLS_CONFIG_FILE usage

commit:
("8841f11a refactor: mbedtls: Remove redundant MBEDTLS_CONFIG_FILE")

Removed usage of MBEDTLS_CONFIG_FILE to make way for mbedtls3 testing,
But later it was pointed out during discussions MBEDTLS_CONFIG_FILE
is a valid build option and currently might be untested in our CI.

So add a simple usage of MBEDTLS_CONFIG_FILE with mbedtls-3 config
file.

Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
Change-Id: Ib0aa07d2307094e21db30764ae8912c9ef3d344e
(cherry picked from commit 31ed053e761d8cbb580b9ee51b11b541e77d1827)
diff --git a/tf_config/fvp-tbb-mbedtls b/tf_config/fvp-tbb-mbedtls
index 8a40e81..129bc9a 100644
--- a/tf_config/fvp-tbb-mbedtls
+++ b/tf_config/fvp-tbb-mbedtls
@@ -1,6 +1,7 @@
 ARM_ROTPK_LOCATION=devel_rsa
 CROSS_COMPILE=aarch64-none-elf-
 GENERATE_COT=1
+MBEDTLS_CONFIG_FILE=\"${tf_root}/include/drivers/auth/mbedtls/mbedtls_config-3.h\"
 PLAT=fvp
 ROT_KEY=plat/arm/board/common/rotpk/arm_rotprivk_rsa.pem
 TRUSTED_BOARD_BOOT=1