libfsimage: add ext4 support for CentOS 5.x

libfsimage: add ext4 support for CentOS 5.x

CentOS 5.x forked e2fs ext4 support into a different package called
e4fs, and so headers and library names changed from ext2fs to ext4fs.
Check if ext4fs/ext2fs.h and -lext4fs work, and use that instead of
ext2fs to build libfsimage. This patch assumes that if the ext4fs
library is present it should always be used instead of ext2fs.

This patch includes a rework of the ext2fs check, a new ext4fs check
and a minor modification in libfsimage to use the correct library.

Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>