cmake: support directory as argument to zephyr_library_amend()

Extend `zephyr_library_amend()` to support an optional directory
argument.
The current `zephyr_library_amend()` works well when used inside a
Zephyr module with same structure, but fails when the macro is called
from Zephyr module integration code is located in a Zephyr
`MODULE_EXT_ROOT` because in this case the CMake code being executed
is not present in the Zephyr module itself, in which case the dir name
creation based on relative to module dir give wrong result.

For this use-case then support a base directory.
This also allows for use-cases in Zephyr modules where the directory
structure matching Zephyr's own structure is placed in a sub-folder.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>