rpm: Fix support for db5 and db6

rpm: Fix support for db5 and db6

The previous attempt to let rpm configuration support both db5 and db6
has a flaw that when the building host provides db6 without its header
the db_create test will false pass. This new patch addresses this issue
by test against the DB_VERSION_MAJOR macro value, which is defined in
both db5 and db6's header.

(From OE-Core rev: 59934080f8311a810e7b5ce82a264d4b9de650ec)

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