mkimage: reduce default total image size to 512 + 256 MiB

because growpart is now run at boot, we can make the base image smaller.
This does not provide any benefit when compressed (the image is still
81M compressed with xz 9), but this should be much faster to write to
the target SD card/USB drive for SBCs, where the throughput is rather
slow. Growpart will inflate the root partition to a reasonable size for
the user. Users who want more flexibility in partitioning should be
doing a chroot/PLATFORMFS install.

mkimage.sh

@@ -65,7 +65,7 @@ usage() {
 	 -b <fstype>    /boot filesystem type (default: vfat)
 	 -B <bsize>     /boot filesystem size (default: 256MiB)
 	 -r <fstype>    / filesystem type (default: ext4)
-	 -s <totalsize> Total image size (default: 2GiB)
+	 -s <totalsize> Total image size (default: 768MiB)
 	 -o <output>    Image filename (default: guessed automatically)
 	 -x <num>       Number of threads to use for image compression (default: dynamic)
 	 -h             Show this help and exit
@@ -128,10 +128,10 @@ case "$PLATFORM" in
         ;;
 esac
 # By default we build all platform images with a 256MiB boot partition
-# formated FAT16, and an approximately 1.88GiB root partition formatted
+# formated FAT16, and an approximately 512MiB root partition formatted
 # ext4.  More exotic combinations are of course possible, but this
 # combination works on all known platforms.
-: "${IMGSIZE:=2G}"
+: "${IMGSIZE:=768M}"
 : "${BOOT_FSTYPE:=vfat}"
 : "${BOOT_FSSIZE:=256MiB}"
 : "${ROOT_FSTYPE:=ext4}"