config: add backup coreboot submodule repositories

this is using the same functionality that was added a few
commits ago, to override the use of "git submodule update"

each coreboot submodule has two repositories defined, with
the second one kicking in if the mail one fails upon cloning.

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

config/submodule/coreboot/default/intel-microcode/module.cfg

@@ -0,0 +1,3 @@
+subrepo="https://review.coreboot.org/intel-microcode.git"
+subrepo_bkup="https://github.com/coreboot/intel-microcode"
+subrev="ece0d294a29a1375397941a4e6f2f7217910bc89"