mkconfig: also create CONFIG defines with BSD sed

Parsing of boards.cfg fails on FreeBSD with the error:

sed: 1: "/=/ {s/=/\t/;q } ; { s/ ...": extra characters at the end
of q command

BSD sed expects commands to be on seperate 'lines', hence it expects
an additional ; before the closing brackets.
BSD sed does not support \t, replaced by literal tab.

Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Cc: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>