Replace variable assignments to shell assignments

This has the wonderful effect of executing the commands once then reusing their output as the variable is needed. Speeds clean build time considerably if `git submodule status` takes a bit on your machine!

Author: nitz

Makefile

@@ -22,8 +22,8 @@ MBR_HEX			 = lib/softdevice/mbr/hex/mbr_nrf52_2.4.1_mbr.hex
 # linker by MCU eg. nrf52840.ld
 LD_FILE      = linker/$(MCU_SUB_VARIANT).ld
 
-GIT_VERSION = $(shell git describe --dirty --always --tags)
-GIT_SUBMODULE_VERSIONS = $(shell git submodule status | cut -d" " -f3,4 | paste -s -d" " -)
+GIT_VERSION != git describe --dirty --always --tags
+GIT_SUBMODULE_VERSIONS != git submodule status | cut -d" " -f3,4 | paste -s -d" " -
 
 # compiled file name
 OUT_FILE = $(BOARD)_bootloader-$(GIT_VERSION)