bhyve: add helper to create a bootorder

Qemu's fwcfg allows to define a bootorder. Therefore, the hypervisor has
to create a fwcfg item named bootorder, which has a newline seperated
list of boot entries. Qemu's OVMF will pick up the bootorder and applies
it.

Add the moment, bhyve's OVMF doesn't support a custom bootorder by
qemu's fwcfg. However, in the future bhyve will gain support for qemu's
OVMF. Additonally, we can port relevant parts from qemu's to bhyve's
OVMF implementation.

Reviewed by: jhb, markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39284