net/ipxe: Update to latest commit

• Make ISO creation optional

• Remove support for IPXE_EMBED and IPXE_TRUST. They can be passed directly via IPXE_MAKE_ARGS in make.conf, Makefile.local or similar instead:

IPXE_MAKE_ARGS= EMBED=${.CURDIR}/ipxe-script

• Add support for setting iPXE build options via IPXE_BUILDCFG

• Enable serial console, ping and poweroff commands