Draft: utils: Add tooling for building sysext images out of bst artifact checkouts

The tooling added here is purposely split in two parts:

• prepare-systext-elements.py: to build and checkout one or more bst elements while making sure this is done appropriately for building sysext images.
• build-sysext-image.py: to build a sysext image out an artifact checkout, and potentially any other arbitrary OS tree.

The reasons for the split are the following:

• Easier management of dependencies.
• Reusability in other scenarios not covered here.
• More flexibility to manage sysext names.

Usage example:

$ ./utils/prepare-systext-elements.py ./sdk/ sdk/gtk.bst sdk/meson.bst ...
$ ./utils/build-sysext-image.py sdk ./sdk/ files/boot-keys/SYSEXT.key files/boot-keys/SYSEXT.crt

See #825 (closed)