build: fix sizing the image during a clean build

The shell call to stat(1) is evaluted when the rule to build the image is
triggered, at which point loader-stripped.elf does not exist yet.  This
causes stat to fail and the build to break.

Fix by moving the creation of loader-stripped.elf to its own target, so
that by the time the recipe is evaluated, the file is known to exist.

build.mk

@@ -160,10 +160,12 @@ boot.bin: arch/x64/boot16.ld arch/x64/boot16.o
 
 image-size = $(shell stat --printf %s loader-stripped.elf)
 
-loader.img: boot.bin loader.elf
-	$(call quiet, dd if=boot.bin of=$@ > /dev/null 2>&1, DD $@ boot.bin)
+loader-stripped.elf: loader.elf
 	$(call very-quiet, cp loader.elf loader-stripped.elf)
-	$(call very-quiet, strip loader-stripped.elf)
+	$(call quiet, strip loader-stripped.elf, STRIP loader.elf)
+
+loader.img: boot.bin loader-stripped.elf
+	$(call quiet, dd if=boot.bin of=$@ > /dev/null 2>&1, DD $@ boot.bin)
 	$(call quiet, dd if=loader-stripped.elf of=$@ conv=notrunc seek=128 > /dev/null 2>&1, \
 		DD $@ loader.elf)
 	$(call quiet, $(src)/scripts/imgedit.py setsize $@ $(image-size), IMGEDIT $@)