grub: add download mirrors

grub: add download mirrors

Add three download mirrors for 'grub', as the Makefile currently defines only
one download location.

@GNU can not be used, as the most recent version of 'grub' is not available
at the general GNU mirrors, and can only be found at the "gnu-alpha" mirrors.

Signed-off-by: Hannu Nyman <hannu.nyman at iki.fi>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45909 3c298f89-4303-0410-b956-a3cf2f4a3e73