system/Config.in: introduce pre-build script

We introduce the concept of a pre-build script that works similar to
the already existing post-build and post-image scripts.

The pre-build script(s) are executed before the build commences. This
allows a user to run some preperatory tasks prior to the build.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

Makefile

@@ -599,6 +599,9 @@ $(BUILD_DIR)/buildroot-config/auto.conf: $(BR2_CONFIG)
 
 .PHONY: prepare
 prepare: $(BUILD_DIR)/buildroot-config/auto.conf
+	@$(foreach s, $(call qstrip,$(BR2_ROOTFS_PRE_BUILD_SCRIPT)), \
+		$(call MESSAGE,"Executing pre-build script $(s)"); \
+		$(EXTRA_ENV) $(s) $(TARGET_DIR) $(call qstrip,$(BR2_ROOTFS_POST_SCRIPT_ARGS))$(sep))
 
 .PHONY: world
 world: target-post-image

system/Config.in

@@ -533,6 +533,16 @@ config BR2_ROOTFS_OVERLAY
 	  They are copied as-is into the rootfs, excluding files ending
 	  with ~ and .git, .svn and .hg directories.
 
+config BR2_ROOTFS_PRE_BUILD_SCRIPT
+	string "Custom scripts to run before commencing the build"
+	default ""
+	help
+	  Specify a space-separated list of scripts to be run before the
+	  build commences.
+
+	  This gives users the opportunity to do board-specific
+	  preparations before starting the build.
+
 config BR2_ROOTFS_POST_BUILD_SCRIPT
 	string "Custom scripts to run before creating filesystem images"
 	default ""