Update regex pattern to allow variable flag name with a single character.
Regression tests have also been updated in `bb.parse` and
`bin/bitbake-selftest -k ParseTest` has been successfully executed.
Eliminate a trailing space as well.
(Bitbake rev: bb9e523291a3cad6e1596ee6a1e715b5e5feba8f)
Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>