u-boot: Split do_configure logic into separate file

Some auxiliary u-boot recipes may need u-boot properly configured
(including *.cfg additions via bbappends) but aren't necessarily
building u-boot itself; to support such situations, here we split the
u-boot do_configure() out of u-boot.inc and into its own
u-boot-configure.inc.

(From OE-Core rev: e55e6fb4983a41f74c0e457bf54bd8dfa5608daa)

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Change-Id: I4941535818effd3e660c227195bd5755d11ef8b4