oeqa/utils/command: fast-path get_bb_var()

oeqa/utils/command: fast-path get_bb_var()

get_bb_var() currently end up calling 'bitbake -e' and parsing the whole
output. However if postconfig isn't set then we can speed this up by
just calling bitbake-getvar.

The complication with failing bitbake-getvar calls is because we need to
be careful to return None instead of the empty string when the variable
doesn't exist.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>