fs/ext2: fix blocks/inodes calculation

With the ROOTFS_TARGET conversion, EXT2_OPTS gets evaluated very early
(before TARGET_DIR is populated with files), so the calculated
blocks/inodes numbers are wrong.

Fix it by moving the calculation to a shell script wrapper around
genext2fs, so it only gets executed just before genext2fs runs.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>