add 'rpath' easyconfig parameter to enable use of RPATH regardless of EasyBuild configuration

[boegel]

We already have an rpath toolchain option that can be used to selectively disable the use of RPATH even if EasyBuild is configured to use RPATH linking via --rpath.

In some cases, it may also be interesting to be able to selectively enforce the use of RPATH, regardless of how EasyBuild was configured; see for example easybuilders/easybuild-easyconfigs#4962 (review)

If the rpath easyconfig parameter is defined as True, RPATH linking is always performed, regardless of the EasyBuild configuration option --rpath.
If --rpath is used, RPATH linking is enabled, unless it's specifically disabled by setting the rpath toolchain option to False (True being the default value).

After implementing this, which was pretty straightforward, I'm starting to have 2nd thoughts about this approach, since having both an rpath toolchain option and easyconfig parameter (on top of the --rpath configuration option...) may lead to confusion...

[boegel]

An alternate approach could be:

• use None as default value for the rpath toolchain option, meaning honor the --rpath configuration setting
• allow disabling using 'rpath': False in toolchainopts (already the case)
• allow hard enabling using 'rpath': True in toolchainopts (regardless of --rpath)

That's probably a better approach... Thoughts @wpoely86, @pescobar, @JackPerdue, @jdonners?

[akesandgren]

"Default" instead of "None" would be less confusing in my opinion but None is easier to code for.
and Enforce (or Enable) vs Disable is probably easier to understand compared to True/False, and can be turned into true/false in the actual code.

[akesandgren]

@boegel Care to fix the conflicts?