drivers: nrfwifi: Fix rebuilding when FW blobs are changes

Whenever FW blobs are updated manually, we need to tell cmake to
auto-build the source files to use the latest firmware without doing a
pristine build.

This adds a custom target to be run with nRF Wi-Fi driver and updates
timestamp of fw_load.c to rebuild whenevr the blob is updated.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>