edk2: fix up UEFI firmware build for QEMU and add secure boot flag
edk2: install split OVMF_CODE and OVMF_VARS images

Both OVMF and QEMU have gained support for splitting the combined OVMF
image into two, now variable storage is its own file.
Merge pull request #949 from marineam/edk2

edk2: fix up UEFI firmware build for QEMU and add secure boot flag