grub2: Add sub package grub-editenv for target installation

grub-editenv allows to modify grub2 environment files. Add a new package
that build grub2 for the target and packs up grub-editenv.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36574 3c298f89-4303-0410-b956-a3cf2f4a3e73