init-install-efi.sh: fix gummiboot entry installation

After selecting the "install" gummiboot option of a Live image we are
seeing boot failure resulting from the gummiboot entries not being
installed correctly. This seems to be a problem in this init-install-efi.sh
script where it incorrectly installs the gummiboot entries into the root
filesystem, not the boot partition. We fix it by installing the entries in
the boot partition.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>