Add chain loader support for loader

Implement simple chain loader in loader; this update does add chain command,
taking device or file as argument to load and start new boot loader.

In case of BIOS, the chain will read the boot block to address 0000:7c00 and
jumps on it. In case of UEFI, the chain command is to be used with efi
application, typically stored in EFI System Partition.

The update also does add simple menu entry, if the variable chain_disk is set.
The value of the variable chain_disk is used as argument for chain loading.

Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D5992