Build: check if specified external image exists

check_* targets check if the required option are given, but do not
check the validity of the argument.  If the specified file does not
exist, let the build fail immediately instead of passing the invalid
file path to tools.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

make_helpers/build_macros.mk

 #
-# Copyright (c) 2015-2017, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -132,6 +132,7 @@ define FIP_ADD_IMG
 .PHONY: check_$(1)
 check_$(1):
 	$$(if $(value $(1)),,$$(error "Platform '${PLAT}' requires $(1). Please set $(1) to point to the right file"))
+	$$(if $(wildcard $(value $(1))),,$$(error '$(1)=$(value $(1))' was specified, but '$(value $(1))' does not exist))
 endef
 
 # FWU_FIP_ADD_PAYLOAD appends the command line arguments required by fiptool
@@ -166,6 +167,7 @@ define FWU_FIP_ADD_IMG
 .PHONY: check_$(1)
 check_$(1):
 	$$(if $(value $(1)),,$$(error "Platform '${PLAT}' requires $(1). Please set $(1) to point to the right file"))
+	$$(if $(wildcard $(value $(1))),,$$(error '$(1)=$(value $(1))' was specified, but '$(value $(1))' does not exist))
 endef
 
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