rrright. ok. these modifications to sdram_init allow it

rrright.  ok.  these modifications to sdram_init allow it
to be compiled and used stand-alone.  sdram initialisation is disabled.
the general idea here is to make this a useful mini-BIOS bootloader,
focussing initially on verilator start-up.  to that end, the first
addition is a SYS_REG_BRAM_BOOTADDR addition to syscon which is the
location where the chain-loading continues (if SPI and SDRAM are not
available, where SDRAM is definitely out at this point)