This require the s25fl128s.vhd flash model and FMF libraries,
which will be built when passed to the Makefile via the
FLASH_MODEL_PATH argument. Otherwise a dummy module is used
which ties MISO to '1'.
The model isn't included as I'm not sure its licence (GPL) is
at this point, but it can be obtained from
FLASH_MODEL_PATH=<path to microspi>/model
Signed-off-by: Benjamin Herrenschmidt <firstname.lastname@example.org>