install: Look for grub2 files on the initramfs, not rootfs

install: Look for grub2 files on the initramfs, not rootfs

Fixes [YOCTO #3870] atom-pc - cannot boot image on netbook after install

The problem here is that grub2 is installed but a grub 1 menu.lst is
created at install time. At boot, grub2 doesn't find a grub.cfg file and
drops to the grub shell.

This happens because the installer is looking for 40_custom (a grub2
file) on the rootfs, but grub2 isn't installed on the rootfs. It exists
in the initramfs. Patching the installer to look on the initramfs
resolves the problem.

Note that the problem may have occurred if grub2 used to be installed on
the rootfs but was later removed. In any case, the installer is HORRIBLE
and really needs to be completely redesigned as part of the deployment
effort. For now, this should get the live image installer limping along
again.

Tested on a Toshiba NB-305.

(From OE-Core rev: 8756a19bd24045d41ad20abb581e7872d0fc9ee6)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: alexandru.c.georgescu@intel.com
Cc: sgw@linux.intel.com
Cc: ross.burton@intel.com
Cc: richard.purdie@intel.com
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>