Use immediate expansion for BUILD_ARCH and BUILD_OS since there is no
point in repeatedly calling os.uname() throughout parsing. This is
worth around 2% of parsing time, small but measurable.
(From OE-Core rev: 03482382b797f3501c3fb0df0fe12bd4e5e51a39)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>