barebox: support multi-image-build image selection

Support optional selection of the built image filename in a
multi-image barebox build. This makes it possible to specify which
image to pick in a multi-image barebox config such as the
am335x_defconfig.

Signed-off-by: Pieter Smith <pieter@boesman.nl>
[Thomas:
 - remove default "", since this is the default for string options
 - rename the prompt from "Barebox image filename" to "Image filename"
 - slightly improve the Config.in help text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/barebox/Config.in

@@ -97,6 +97,16 @@ config BR2_TARGET_BAREBOX_CONFIG_FRAGMENT_FILES
 	  A space-separated list of configuration fragment files,
 	  that will be merged to the main Barebox configuration file.
 
+config BR2_TARGET_BAREBOX_IMAGE_FILE
+	string "Image filename"
+	help
+	  Name of the generated barebox image, which will be copied to
+	  the images directory.
+
+	  If left empty, defaults to:
+	  - barebox.bin for barebox versions older than 2012.10.
+	  - barebox-flash-image for later versions.
+
 config BR2_TARGET_BAREBOX_BAREBOXENV
 	bool "bareboxenv tool in target"
 	help

boot/barebox/barebox.mk

@@ -91,8 +91,12 @@ define BAREBOX_BUILD_CMDS
 	$(BAREBOX_BUILD_CUSTOM_ENV)
 endef
 
+BAREBOX_IMAGE_FILE = $(call qstrip,$(BR2_TARGET_BAREBOX_IMAGE_FILE))
+
 define BAREBOX_INSTALL_IMAGES_CMDS
-	if test -h $(@D)/barebox-flash-image ; then \
+	if test -n "$(BAREBOX_IMAGE_FILE)"; then \
+		cp -L $(@D)/$(BAREBOX_IMAGE_FILE) $(BINARIES_DIR) ; \
+	elif test -h $(@D)/barebox-flash-image ; then \
 		cp -L $(@D)/barebox-flash-image $(BINARIES_DIR)/barebox.bin ; \
 	else \
 		cp $(@D)/barebox.bin $(BINARIES_DIR);\