arm/edk2-firmware: rationalise $S

Currently edk2-firmware sets S to WORKDIR/git/ and unpacks edk2 into
WORKDIR/git/edk2. This isn't idiomatic and breaks tooling such as devtool
which expects a git fetcher to unpack a git repository to $S.

Move the edk2 unpack to $S directly, and update the recipe where required.

Change-Id: Ib9571032c14a01864860e2eb5a04ed73dd382710
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>