tools: Calculate EFI ESP FAT FS size based on contents.

This requires switching to the dosfstools from alpine:edge since neither the
busybox nor alpine:3.5 dosfstools supports the -C option (in fact alpine:3.5
only has mkfs.fat and not mkfs.vfat).

The 511k slack seems like a lot to me, but 256k was somehow not enough.

Fixes #1304.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

tools/mkimage-iso-efi/Dockerfile

@@ -1,8 +1,9 @@
-FROM alpine:3.5
+FROM alpine:edge
 
 RUN \
   apk update && apk upgrade && \
   apk add --no-cache \
+  dosfstools \
   libarchive-tools \
   binutils \
   gummiboot \

tools/mkimage-iso-efi/make-efi

@@ -37,14 +37,15 @@ objcopy \
 
 # create a ISO with a EFI boot partition
 mkdir -p iso
-dd if=/dev/zero of=iso/efi.raw bs=1024 count=78000
-mkfs.vfat iso/efi.raw
-
+# Stuff it into a FAT filesystem, making it as small as possible.  511KiB
+# headroom seems to be enough; (x+31)/32*32 rounds up to multiple of 32.
+mkfs.vfat -v -C iso/efi.raw \
+        $(( ($(stat -c %s "mobylinux.efi") / 1024 + 511) \
+		/ 32 * 32 ))
 echo "mtools_skip_check=1" >> /etc/mtools.conf && \
 mmd -i iso/efi.raw ::/EFI
 mmd -i iso/efi.raw ::/EFI/BOOT
 mcopy -i iso/efi.raw mobylinux.efi ::/EFI/BOOT/BOOTX64.EFI
-
 xorriso -as mkisofs \
 	-R -f -e efi.raw -no-emul-boot -o mobylinux-efi.iso iso