Add precompiler flag to disable sdram usage

[ohmtech-rdi]

This PR adds the erb_SDRAM_USE_FLAG precompiler flag configuration, on by default, to deactive SDRAM usage when the module doesn't use SDRAM.

This allows to save production costs, when the Daisy board is available without the SDRAM chip, which is the case for both the Daisy Seed and Daisy Patch Submodule (aka "Memory: 1MB").

To not use SDRAM, one would add in their module erbb definition:

define erb_SDRAM_USE_FLAG=false