base-files: replace fgrep with grep -F

fgrep is deprecated and replaced by grep -F. The latter is used
throughout the tree whereas this is the only usage of the former.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

package/base-files/files/etc/profile

@@ -3,7 +3,7 @@
 [ -f /etc/banner ] && cat /etc/banner
 [ -n "$FAILSAFE" ] && cat /etc/banner.failsafe
 
-fgrep -sq '/ overlay ro,' /proc/mounts && {
+grep -Fsq '/ overlay ro,' /proc/mounts && {
 	echo 'Your JFFS2-partition seems full and overlayfs is mounted read-only.'
 	echo 'Please try to remove files from /overlay/upper/... and reboot!'
 }