tbbr-tools: enable override TRUSTED_KEY_CERT

Platforms, which requires additional images to be
verified using TBBR; such that their key certificate
is tied to TRUSTED_KEY_CERT.

For such platforms, if make commands runs twice:
 - Once with targets as bl2 & fip.bin, and
 - Again to build the target as the additional image.

then, if path to the TRUSTED_KEY_CERT varies in the
makefile with make-target of the additional image, then
there would be two location where "trusted_key.crt" will
be created.

This patch helps overriding the TRUSTED_KEY_CERT from any .mk
in the platform's makefile structure.
Signed-off-by: Pankaj Gupta <pankaj.gupta@nxp.com>
Change-Id: I775a2c409035504b21b0bbe5a4f9046898163eed

make_helpers/tbbr/tbbr_tools.mk

 #
-# Copyright (c) 2015-2020, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2015-2021, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -33,7 +33,7 @@
 #
 
 # Certificate generation tool default parameters
-TRUSTED_KEY_CERT	:=	${BUILD_PLAT}/trusted_key.crt
+TRUSTED_KEY_CERT	?=	${BUILD_PLAT}/trusted_key.crt
 FWU_CERT		:=	${BUILD_PLAT}/fwu_cert.crt
 
 # Default non-volatile counter values (overridable by the platform)