buildroot

build: fix umask test

Some shells' builtin umask does not print 2 leading 0's for the umask.
Switching to bash is done anyway.
This patch switches to bash before the umask test.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>