mkimage: allow -l to work on block devices on Linux

When "mkimage -l" was run on a block device it would fail with
erroneous message, because fstat reports a size of zero for those:

 mkimage: Bad size: "/dev/sdb4" is not valid image

This patch identifies the "is a block device" case and reports it as
such, and if it knows how to determine the size of a block device on
the current OS, proceeds.

As shown in
http://www.mit.edu/afs.new/sipb/user/tytso/e2fsprogs/lib/blkid/getsize.c


this is no portable task, and I only handled the case of a modern
Linux kernel, which is what I can test.

Signed-off-by: Yann Dirson <yann@blade-group.com>