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

(cherry picked from commit 676b7d077c2c5f548334cea7fccfbfb5d097c9df)