cmake: allow to link external libraries with --whole-archive

[aescolar]

Allow apps or boards to define a set of external libraries to be linked into the zephyr binary,
but inside the -Wl,--whole-archive part of the link command.
For ex. an app may want to link the libraries
/path/lib_a.a /path/lib_b.a & /path/lib_c.a
where these libraries may use each other.

Background:
Currently it is only possible to add them after the --no-whole-archive part of the link command.
Which means the linker will ignore any symbol inside these libraries it has not seen some use for yet.
So, if for ex. lib_b is the first one using some symbol in lib_a but lib_a was listed first, the symbol won't be found.
[The old fashion solution, is to order the libraries carefully and list them as many times as needed, but that is what the --whole-archive linker option is meant to avoid]

[SebastianBoe]

We have discussed this offline. There are several ways to achieve the same effect, but none that seem
cleaner than adding them to whole-archive.

Solution proposal:

Introduce a "Zephyr external library" concept to the CMake extensions.

These libraries are conceptually Zephyr libraries (so they are for instance added to the list of libraries to be --whole-archive'd), but they have a property set on them so that they are allowed to be source-less (Zephyr libraries are normally not allowed to be source-less).