Extend/fix support for extlinux bootloader

Some extlinux packages install mbr.bin under /usr/lib/extlinux or
/usr/lib/EXTLINUX so tese directories need to be checked as well.
Also, there are packages that create a /boot/extlinux directory
which needs to be taken into account. Finally, commit
bccffc8bfdfea4c452a5558a27f4d16da9cab6c7 dropped the /boot prefix
from the kernel and initrd references in the ext/syslinux config
file which is simply a bug that needs fixing.

Change-Id: Idb071c9b18ff557b0f0f76d0d754536f2deca904
Closes-Bug: 1335042
Merge "Extend/fix support for extlinux bootloader"