mkbootimg: use make_ext4fs to avoid loop mount
mkbootimg: use make_ext4fs to avoid loop mount

This patch changes the mkbootimg.sh script if the boot partition is
ext4 filesystem. The make_ext4fs tool can be used to make a ext4
file system image from a local directory.

Change-Id: I7b9d8e5c9c8d0d9599e752366aff8ef788a2e823
Signed-off-by: Chanho Park <8ec285a53e21b773fd914e03c50e1b6431bc9aa8@samsung.com>

SamsungARTIK/build-artik