When using GREP_OPTIONS to supply default options to grep, the buildsystem might get broken (For example adding --color=always breaks it)
This patch will empty the GREP_OPTIONS to prevent the described (and any other) problems related to GREP_OPTIONS

Signed-off-by: Maarten Bezemer <m.m.bezemer@utwente.nl>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@22443 3c298f89-4303-0410-b956-a3cf2f4a3e73