BIOS breaks without DDR main ram

[mithro]

This use to build fine on older versions of LiteX / LiteDRAM.

make[1]: Leaving directory `/home/travis/build/mithro/litex-buildenv/build/basys3_base_lm32/software/libnet'
make[1]: Entering directory `/home/travis/build/mithro/litex-buildenv/build/basys3_base_lm32/software/bios'
make[1]: warning: jobserver unavailable: using -j1.  Add `+' to parent make rule.
 CC       isr.o
 CC       sdram.o
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c: In function 'memtest_bus':
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c:544:49: error: 'MAIN_RAM_BASE' undeclared (first use in this function); did you mean 'SRAM_BASE'?
  volatile unsigned int *array = (unsigned int *)MAIN_RAM_BASE;
                                                 ^~~~~~~~~~~~~
                                                 SRAM_BASE
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c:544:49: note: each undeclared identifier is reported only once for each function it appears in
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c: In function 'memtest_data':
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c:596:49: error: 'MAIN_RAM_BASE' undeclared (first use in this function); did you mean 'SRAM_BASE'?
  volatile unsigned int *array = (unsigned int *)MAIN_RAM_BASE;
                                                 ^~~~~~~~~~~~~
                                                 SRAM_BASE
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c: In function 'memtest_addr':
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c:636:49: error: 'MAIN_RAM_BASE' undeclared (first use in this function); did you mean 'SRAM_BASE'?
  volatile unsigned int *array = (unsigned int *)MAIN_RAM_BASE;
                                                 ^~~~~~~~~~~~~
                                                 SRAM_BASE
At top level:
/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/sdram.c:15:13: warning: 'cdelay' defined but not used [-Wunused-function]
 static void cdelay(int i)
             ^~~~~~
make[1]: *** [sdram.o] Error 1
make[1]: Leaving directory `/home/travis/build/mithro/litex-buildenv/build/basys3_base_lm32/software/bios'
Traceback (most recent call last):
  File "./make.py", line 164, in <module>
    main()
  File "./make.py", line 148, in main
    vns = builder.build(**dict(args.build_option))
  File "/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/integration/builder.py", line 157, in build
    self._generate_software(not self.soc.integrated_rom_initialized)
  File "/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/integration/builder.py", line 142, in _generate_software
    subprocess.check_call(["make", "-C", dst_dir, "-f", makefile])
  File "/home/travis/build/mithro/litex-buildenv/build/conda/lib/python3.6/subprocess.py", line 291, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['make', '-C', '/home/travis/build/mithro/litex-buildenv/build/basys3_base_lm32/software/bios', '-f', '/home/travis/build/mithro/litex-buildenv/third_party/litex/litex/soc/software/bios/Makefile']' returned non-zero exit status 2.