This commit de-duplicates the redundant dummy implementations and
to some degree also the 'lx_emul.c' implementations while also
decluttering the various 'dep.list' files.
The code is moved into 'src/lib/pc/lx_emul' where it becomes part
of the 'pc_lx_emul' library.
Fixes#4500.
This commit contains:
* Minimal Linux kernel target: pc_linux
* Library to generate a Linux build directory, config, generated headers
* API depot package
The actual work was provided by Josef Soentgen.
Ref genodelabs/genode#4416
