Replace page_allocator.c_inc mechanism with header file

`page_allocator.c` could have a conditional `.h` include depending on whether the `.c` is compiled in a uvisor context or in unsupported. The unsupported header file does not need to be included in the supported tree and vice versa. Potentially the header file name can be identical for bot supported and unsupported (`page_allocator_config.h`) and have different content for supported/unsupported.

Requires support from build system `UVISOR_UNSUPPORTED` folder.

Start with page_allocator.c and the UVISOR_UNSUPPORTED folder, then trace how the current page_allocator.c_inc mechanism is included in supported and unsupported builds. Done means replacing that mechanism with conditional page_allocator_config.h headers and adding the required build-system support for UVISOR_UNSUPPORTED.

Written by the indexing model from the issue text.