sysutils/uefi-edk2-bhyve*: Update and migrate to Python 3

o remove dependency on the now obsolete Python 2.7.
o sysutils/uefi-edk2-bhyve now uses the GH/tianocore/edk2 upstream repo.

Use commit 2e1e8c35f3178df95d79da81ac6deec242da74c2 from master on
2021-02-14.

o sysutils/uefi-edk2-bhyve-csm is no longer a slave port, since it

continues to use the GH/freebsd/uefi-edk2 repo.

o sysutils/uefi-edk2-bhyve-devel is no longer required, so delete it.
o Update the port Makefiles to follow the style in sysutils/edk2.

Reviewed by: grehan
Approved by: woodsb02
Differential Revision: https://reviews.freebsd.org/D27230