build: image: posix compatibility cut v head

Replace 2 instances of non posix use of 'head' with posix compliant
'cut'.

'head -c n' cuts 'n' bytes from the passed string and happens to work on
Linux & Mac OS X even though '-c' is not posix.

'head --bytes n' does the same thing and happens to work on linux but
not on Mac OS X and is also not posix.

'cut -b1-8' cuts the first 8 bytes from the passed string and is posix
compliant, hence works on Linux & Mac OS X.

Our usage of 'head --bytes' was particularly unfortunate since it was
used to calculated the RootFS UUID passed to grub - the net result being
a non-functioning system waiting for the root file system to appear.

Thanks to karlp, ynezz & others for pointers on solving this.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

include/image.mk

@@ -30,7 +30,7 @@ param_get_default = $(firstword $(call param_get,$(1),$(2)) $(3))
 param_mangle = $(subst $(space),_,$(strip $(1)))
 param_unmangle = $(subst _,$(space),$(1))
 
-mkfs_packages_id = $(shell echo $(sort $(1)) | mkhash md5 | head -c 8)
+mkfs_packages_id = $(shell echo $(sort $(1)) | mkhash md5 | cut -b1-8)
 mkfs_target_dir = $(if $(call param_get,pkg,$(1)),$(KDIR)/target-dir-$(call param_get,pkg,$(1)),$(TARGET_DIR))
 
 KDIR=$(KERNEL_BUILD_DIR)
@@ -44,7 +44,7 @@ IMG_PREFIX_VERCODE:=$(if $(CONFIG_VERSION_CODE_FILENAMES),$(call sanitize,$(VERS
 IMG_PREFIX:=$(VERSION_DIST_SANITIZED)-$(IMG_PREFIX_VERNUM)$(IMG_PREFIX_VERCODE)$(IMG_PREFIX_EXTRA)$(BOARD)$(if $(SUBTARGET),-$(SUBTARGET))
 IMG_ROOTFS:=$(IMG_PREFIX)-rootfs
 IMG_COMBINED:=$(IMG_PREFIX)-combined
-IMG_PART_SIGNATURE:=$(shell echo $(SOURCE_DATE_EPOCH)$(LINUX_VERMAGIC) | mkhash md5 | head --bytes 8)
+IMG_PART_SIGNATURE:=$(shell echo $(SOURCE_DATE_EPOCH)$(LINUX_VERMAGIC) | mkhash md5 | cut -b1-8)
 
 MKFS_DEVTABLE_OPT := -D $(INCLUDE_DIR)/device_table.txt