build: image: make image padding OS agnostic

Image padding with dd needs to be done in an OS independent way as not
all dd understand capital letter unit specifiers, and not all understand
lowercase either.

Fixes: 97833a57ef ("build: image: add pad-to and pad-rootfs-squashfs helpers")
Reported-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

include/image.mk

@@ -185,8 +185,10 @@ define Image/pad-to
 	mv $(1).new $(1)
 endef
 
+ROOTFS_PARTSIZE=$(shell echo $$(($(CONFIG_TARGET_ROOTFS_PARTSIZE)*1024*1024)))
+
 define Image/pad-root-squashfs
-	$(call Image/pad-to,$(KDIR)/root.squashfs,$(if $(1),$(1),$(CONFIG_TARGET_ROOTFS_PARTSIZE)M))
+	$(call Image/pad-to,$(KDIR)/root.squashfs,$(if $(1),$(1),$(ROOTFS_PARTSIZE)))
 endef
 
 # $(1) source dts file
@@ -284,11 +286,9 @@ define Image/mkfs/ubifs
 		-o $@ -d $(call mkfs_target_dir,$(1))
 endef
 
-E2SIZE=$(shell echo $$(($(CONFIG_TARGET_ROOTFS_PARTSIZE)*1024*1024)))
-
 define Image/mkfs/ext4
 	$(STAGING_DIR_HOST)/bin/make_ext4fs \
-		-l $(E2SIZE) -b $(CONFIG_TARGET_EXT4_BLOCKSIZE) \
+		-l $(ROOTFS_PARTSIZE) -b $(CONFIG_TARGET_EXT4_BLOCKSIZE) \
 		$(if $(CONFIG_TARGET_EXT4_RESERVED_PCT),-m $(CONFIG_TARGET_EXT4_RESERVED_PCT)) \
 		$(if $(CONFIG_TARGET_EXT4_JOURNAL),,-J) \
 		$(if $(SOURCE_DATE_EPOCH),-T $(SOURCE_DATE_EPOCH)) \