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>
diff --git a/make_helpers/build_macros.mk b/make_helpers/build_macros.mk
index a4fbc5a..4e69b02 100644
--- a/make_helpers/build_macros.mk
+++ b/make_helpers/build_macros.mk
@@ -1,5 +1,5 @@
 #
-# 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 @@
 .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 @@
 .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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