bitbake.conf: set BB_DEFAULT_UMASK using ??=

bitbake.conf: set BB_DEFAULT_UMASK using ??=

Currently, there's no way for the user's site.conf, local.conf or
similar to set BB_DEFAULT_UMASK, because those files are included by
bitbake.conf prior to the unconditional assignment of
BB_DEFAULT_UMASK. To make that possible, use a weak default assignment
instead. This is also consistent with most other variable assignments
in the lower half of bitbake.conf.

I believe the risk of a regression is very small; it would require
something like somebody having a definition of BB_DEFAULT_UMASK in a
local configuration file, and having been relying on that _not_ taking
effect.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>