buildroot

Makefile: fix source/source-check/external-deps with O=

Based on patch by Will Wagner <will_wagner@carallon.com>.

Now that the .config is located in the output directory when O=<dir>
is used, we have to pass the O= option to make for
source/source-check/external-deps to get them to look in the right place
for the .config.

Fix it by introducing an EXTRAMAKEARGS variable and use it whenever we
call back into the toplevel Makefile.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>