cmake: support multiple entries in board.cmake

Currently there is no way to support running a board on multiple
emulation platforms nor to choose a desired emulation platform for the
simulation to be run on. This commit introduces a new
SUPPORTED_EMU_PLATFORMS list, which defines available emulation
platforms for a given board.

Fixes #12375.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>