update/repos: concatenate multiple revision files

With this change, it's still possible to have a single
file at config/git/revisions, but this has been scrapped.

Instead, multiple files now exist under config/git/ with
the same modules declared, but the files are separated
logically. List of files under config/git:

* bios_extract
* biosutilities
* coreboot
* flashrom
* grub (gnulib also defined here)
* me_cleaner
* memtest86plus
* seabios
* serprog (multiple projects defined)
* u-boot
* uefitool

The rationale behind this change is simple: in the future,
we will stop relying on build systems within imported
projects for the import of git submodules. Instead, we
will handle them directly in lbmk.

Additionally, a Linux payload is planned for Libreboot, made
easier by the recent audit (script handle/make/config makes
it easy to integrate Linux, and handle cross-compilers for
userland utilities); a "linux" file under config/git/ could
also define rules for each project besides linux, such as
musl libc, busybox and other utilities.

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

config/git/u-boot

@@ -0,0 +1,6 @@
+{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
+}