[Doc] Update amiberry manpage

[giantclambake]

Just updating this manpage, to keep it aligned with amiberry changes/enhancements, on the incline to the v7.0 release.

*added env_variable descriptions
*added a couple of examples
*added reference to cmdline invocation enhancements
*added reference to amberry.portable file

As always, please read/review ;)

TIA

amiberry.1.gz

[midwan]

Thanks.
There were a few minor typos which I fixed, otherwise looks good.