u-boot: Add support for UBOOT_CONFIG_FRAGMENTS and UBOOT_FRAGMENTS

u-boot: Add support for UBOOT_CONFIG_FRAGMENTS and UBOOT_FRAGMENTS

There are config fragments that live in the u-boot source tree.
UBOOT_FRAGMENTS will globally add a set of fragments to the configure.
UBOOT_CONFIG_FRAGMENTS makes use of the overhauled config flow to add
support for specifying alternative fragments for each variation build.

Signed-off-by: Ryan Eatmon <reatmon@ti.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>