arch-handbook: Update boot chapter

A lot has changed in the code since RELEASE 10.0 in 2014, when this
document last received a major content change.

One significant change is in the path to the boot folder, ie
src/sys/boot has become src/stand/.

Another change is that various code blocks have had their sample texts
updated, such as the dmesg now looking like it does on a new install.

Similarly, the assembly code has been updated with the relevant sections
from the source tree. The spacing has been changed to be maximally
compatible with the original version.

Reviewed by: imp (src), Pau Amma <pauamma@gundo.com>
Pull Request: https://github.com/freebsd/freebsd-doc/pull/60