Add more info. to the build/install log file

[paulo-ferraz-oliveira]

Is your feature request related to a problem?

Kinda. It'd ease debugging, I believe.

Describe the feature you'd like

kerl's build/install log should maybe contain all KERL_ prefixed env. variables, instead of just the KERL_BUILD_-prefixed ones.

e.g. it'd probably be informative to have KERL_DOC_TARGETS there, but it's being filtered out.

And since other options control the build and install process, I'm also wondering if the log should be extended to include:

* KERL_BUILD_DOCS=yes
* ...

and then

Install options:
* KERL_BUILD_DOCS=yes
...

since we might change our configuration between one operation and the other one (though this might be a rarer occurrence).

Describe alternatives you've considered

None.

Additional context

Not available.