boot/efi: Prefer nm to objdump

Both objdump and nm are equally capable of reporting undefined symbols.

This gets us a step closer to building without binutils as we have an nm
implementation from ELF Tool Chain.

Reviewed by: bdrewery
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5613