sysutils/edk2: Add bhyve as a new FLAVOR of edk2

Instead of having a separate sysutils/uefi-edk2-bhyve port, add bhyve
as a FLAVOR of sysutils/edk2 and delete sysutils/uefi-edk2-bhyve.

To support existing users who depend on firmware in
/usr/local/share/uefi-firmware create compatibility symlinks.

Update sysutils/bhyve-firmware to depend on edk2@bhyve.

Differential Revision: https://reviews.freebsd.org/D35148
Reviewed by: manu
Approved by: manu