spi flash: remove initialization and move erase to software

Using the volatile configuration register to configure a flash chip
is a bad idea. The problem is that if the FPGA is reset, the flash
may be in a state inconsistent with what the FPGA requires to boot.

The correct solution is to configure the non-volatile configuration
register on the chip to what the FPGA expects on power-on. Then use
these same settings inside the flash core.

Going this route makes it necessary for software to be able to set
the non-volatile configuration register. Rather than making the core
even more complicated than it is, I have elected to add a FIFO which
software can fill to issue custom SPI commands. Since erase can only
be done from software anyway, I removed this code and let erase use
the custom command FIFO.