merge coreboot/u-boot download logic to one script

they are fundamentally the same, in an lbmk context.

they are downloaded in the same way, and compiled in
the same way!

(Kconfig infrastructure, board-specific code, the way
submodules are used in git, etc)

~200 sloc reduction in resources/scripts

the audit begins

Signed-off-by: Leah Rowe <leah@libreboot.org>

resources/git/revisions

@@ -13,6 +13,13 @@
 	bkup_url: https://github.com/coreboot/coreboot.git
 }
 
+{u-boot}{
+	rev: HEAD
+	loc: u-boot/u-boot
+	url: https://source.denx.de/u-boot/u-boot.git
+	bkup_url: https://github.com/u-boot/u-boot.git
+}
+
 {grub}{
 	rev: 7a994c87f571ac99745645be0bdde9827297321a
 	loc: grub