Hook ispfw into build.

Note that this is not yet incorporated into the kernel because:

1) this would be a 3MB object.  Even if this is compressed, it would
slow down kernel load for all users.  For same reason this is not
incorporated into loader.conf by default.

2) our shipped version of firmware could be old and loading it is
not always beneficial.

With this change, it is now possible to add ispfw_load="YES" through
the GUI to enable loading of the firmware.

Ticket: #3537