bsdinstall: create /efi/boot directory in ESP

If the installer is creating a new ESP, then this directory will not
exist and the subsequent cp will fail silently. This is usually of no
consequence if /efi/freebsd/loader.efi is set up correctly.

Reviewed by: imp
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D28176