boards: qemu_x86_tiny: enable support for coverage

This adds the bits so that we can use qemu_x86_tiny for
coverage, as this is currently the only board that can do
demand paging.

This uses the board revision as a way to specify the RAM
size as coverage requires more memory available to store
the coverage data. By piggybacking onto board revision,
this avoids adding another board config just for coverage.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>