Making EDK2 into a module will aid in allowing it to be rebuilt (linuxboot#359)
and patched by the build system and to be reproducible (linuxboot#403). It also
provides somewhat of logical separation from Linuxboot.

Further, Linuxboot builds have not been working with HEADS for a
while because it hasn't been pinned to a commit, and therefore it changes
as the repository is updated. This commit pins Linuxboot to a commit
that has shown to work.

Linuxboot commit: f0ea0af8609e235631f1fcb65a4f7eb69f28ea26
EDK2 commit:      b7c299e4799012e8a20958a68693ab95b7626aa9

Addresses build errors in Ubuntu 20.04 (linuxboot#978)