Add chain loader support for loader

[tsoome]

Summary

This update adds chain command to support simple chain loader. chain command accepts disk
device name as argument, then attempts to read MBR or VBR block (512B) into memory
and will jump to it. As an update to initial version, the bios chain is also allowed to read the bootcode from the file.

In addition, if variable "chain_disk" is set to disk device name, beastie menu will show an
ChainLoader menu entry for quick access to chain loading.

The UEFI chain command is intended to load efi application from ESP.

Test Plan

This update can only be tested in multiboot setups, to load another operating system.

The bios chain is in use in loader illumos port and appears to function quite nicely. The uefi chain I have tested only with loader boot1.efi (chaining illumos from freebsd:)

[tsoome]

Updated to revision 298901.

[tsoome]

Update to rev 301412.
Reserve extra space in relocator.

[tsoome]

Updated to revision 302853.

[tsoome]

svn update to 313091.

[allanjude]

Can you add some instructions or a man page explaining how to use it? (and how one would test it)

[tsoome]

In D5992#195948, @allanjude wrote:

Can you add some instructions or a man page explaining how to use it? (and how one would test it)

from OK prompt: chain disk1: or chain disk1p1:

if the same device is set as value for chain_disk="disk1:" the menu would appear (hopefully). I guess the man update would be good indeed, I'll set note for myself:)

[tsoome]

Updated to revision 318337.

[tsoome]

allow to read boot code from file

[tsoome]

bios chain is missing size, add efi chain.

[tsoome]

menu entry has extra l.

[cem]

head/sys/boot/i386/loader/chain.c

111–113 ↗

(On Diff #29731)

This is totally bogus. uintptr points at relocater_data, which is a uint32_t, not an array type.

Coverity rightly warns about it in CID 1376405 — Memory corruption.

[tsoome]

head/sys/boot/i386/loader/chain.c

111–113 ↗

(On Diff #29731)

Addressed in https://reviews.freebsd.org/D11321, thank you!