build: enable adding MbedTLS files for platform

The platform.mk can add extra MbedTLS source files to LIBMBEDTLS_SRC.

Change-Id: Ida9abfd59d8b02eae23ec0a7f326db060b42bf49
Signed-off-by: Mate Toth-Pal <[email protected]>

Author: matetothpal

drivers/auth/mbedtls/mbedtls_common.mk

@@ -22,7 +22,7 @@ $(eval $(call add_define,MBEDTLS_CONFIG_FILE))
 MBEDTLS_SOURCES	+=		drivers/auth/mbedtls/mbedtls_common.c
 
 
-LIBMBEDTLS_SRCS		:= $(addprefix ${MBEDTLS_DIR}/library/,	\
+LIBMBEDTLS_SRCS		+= $(addprefix ${MBEDTLS_DIR}/library/,	\
 					aes.c 					\
 					asn1parse.c 				\
 					asn1write.c 				\