Config: Don't interpret file names as regular expressions

Config: Don't interpret file names as regular expressions

When we are searching for changes in rootfiles, we walk through each
file that we have found in the build and check if it exists in the
rootfile. That check interpreted filenames as regular expressions which
caused a problem in the case of "/usr/bin/[".

This patch changes that grep will only search for an exact string match
(-F) and the string must be the entire line (-x).

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Reviewed-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>