[rtl] Cleanup/update assertions and "auto-configuration"

[stnolting]

Invalid generic configurations (for example a cache block size that is not a power of two) will no longer result in an error assertion. Instead, the invalid configuration is auto-adjusted and the user is informed with a warning. For example:

  assert not ((nblocks_valid_c = false) or (block_size_valid_c = false)) report
    "[NEORV32] Auto-adjusting invalid d-cache size configuration(s)." severity warning;