kern.opts.mk: Remove EFI from riscv BROKEN_OPTIONS

This discrepancy previously caused make makeman to warn
riscv/riscv64: ignoring duplicate option EFI. Make the kernel EFI
option consistent with EFI's per-arch defaults in src.opts.mk to remove
the warning. Note that we do build EFI boot programs for RISC-V.

This is a NFC because the only component gated by the kernel's EFI
option is sys/modules/efirt, which is also inside of a MACHINE_CPUARCH
block that excludes riscv64 anyway.

Reviewed by: mhorne
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49549