modules/musl-cross: use echo from the PATH to support NixOS

NixOS doesn't have a traditional FHS where echo is available at
`/bin/echo`. Instead, we must rely on the PATH for any
distribution-managed utilities. Reverses
#106.

Author: rbreslow

modules/musl-cross

@@ -34,8 +34,8 @@ else
 # Force a full build of the cross compiler for target platform
 # No need to build i386 for x86 since coreboot uses its own compiler
 musl-cross_configure := \
-	/bin/echo -e >> Makefile 'musl-target:' ; \
-	/bin/echo -e >> Makefile '\t$$$$(MAKE) TARGET="$(MUSL_ARCH)-linux-musl" install' ;
+	echo -e >> Makefile 'musl-target:' ; \
+	echo -e >> Makefile '\t$$$$(MAKE) TARGET="$(MUSL_ARCH)-linux-musl" install' ;
 
 CROSS_PATH ?= $(pwd)/crossgcc/$(CONFIG_TARGET_ARCH)