allow CROSS or MUSL_DIR to be set on the command line so that an external cross compiler can be used (issue #162)

Makefile

@@ -73,15 +73,16 @@ $(shell mkdir -p "$(initrd_lib_dir)" "$(initrd_bin_dir)")
 SHELL := /bin/bash
 .SHELLFLAGS := -o pipefail -c
 
-# If musl-libc is being used in the initrd, set the heads_cc
-# variable to point to it.
+# Include the musl-cross module early so that $(CROSS) will
+# be defined prior to any other module.
+include modules/musl-cross
+
 musl_dep	:= musl
 heads_cc	:= $(INSTALL)/bin/musl-gcc \
 	-fdebug-prefix-map=$(pwd)=heads \
 	-gno-record-gcc-switches \
 	-D__MUSL__ \
 
-CROSS		:= $(build)/../crossgcc/x86_64-linux-musl/bin/x86_64-musl-linux-
 CROSS_TOOLS_NOCC := \
 	AR="$(CROSS)ar" \
 	LD="$(CROSS)ld" \

modules/musl-cross

@@ -1,14 +1,42 @@
-modules-y += musl-cross
+ifeq "$(MUSL_CROSS_ONCE)" ""
+MUSL_CROSS_ONCE := 1
 
+
+# Allow the path to the musl-cross directory to be passed on
+# the environment so that the full path doens't need to be spelled out
+ifneq "$(MUSL_DIR)" ""
+CROSS := $(MUSL_DIR)/crossgcc/x86_64-linux-musl/bin/x86_64-linux-musl-
+endif
+
+ifneq "$(CROSS)" ""
+
+# check that $(CROSS)gcc exists or else things just won't work
+ifneq "y" "$(shell [ -x '$(CROSS)gcc' ] && echo y)"
+$(error $(CROSS)gcc does not exist - can not build
+endif
+
+# The cross compiler has already been built, so the musl-cross target
+# is a NOP.
+musl-cross.intermediate:
+
+else
+
+# Force a full build of the cross compiler
+
+modules-y += musl-cross
 musl-cross_version := git
 musl-cross_dir := musl-cross-$(musl-cross_version)
 musl-cross_repo := https://github.com/GregorR/musl-cross
 
-musl-cross_output := ../../crossgcc/x86_64-linux-musl/bin/x86_64-linux-musl-gcc
-
 musl-cross_configure := \
 	/bin/echo -e > Makefile \
 	'$(musl-cross_output):\n\tCC_BASE_PREFIX="$(pwd)/crossgcc" ./build.sh'
 
+CROSS := $(build)/../crossgcc/x86_64-linux-musl/bin/x86_64-musl-linux-
+musl-cross_output := $(CROSS)gcc
+
+endif
+
 musl-cross_target :=
 
+endif