scripts: mkmakefile: set umask before calling BR's makefile

Small optimization so we don't have another 'make' level (caused by the
umask fix) when running the generated makefile.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>