Fix git version in Makefile on older versions of git (fixes ginge#63)

Author: rigel314

Makefile

@@ -50,6 +50,9 @@ QEMU ?= qemu-pebble
 # output directory
 BUILD = build
 
+GIT_SHA := $(shell git rev-parse HEAD | sed -re 's/^(.......).*/\1/')
+GIT_DIRTY := $(shell [ $$(git describe --dirty --always|grep -- '-dirty$$') ] && echo -dirty)
+
 all: $(PLATFORMS)
 
 # Build rules for each platform, evaluated below.