Missing optionalization for ROMFS about elks

[mfld-fr]

Before revising the code, status reported by Juan:
BEFORE ROMFS patch:
System is 106544 B (48976 B code, 6440 B data and 51128 B bss)
AFTER ROMFS patch:
System is 106764 B (49008 B code, 6448 B data and 51308 B bss)

After setting the size of the block device array according to configuration, and a little clean-up in mem.c, figures go down:
System is 106596 B (49008 B code, 6436 B data and 51152 B bss)

Additional code is mainly the new error message in super.c when failed to open the root device.