Add unbanked support for mod2gbt (GBDK Legacy)

[bbbbbr]

Hi,

Here's a PR if you'd like it. It could be useful for other GBDK folks.

The Issue:

In GBDK (2.9x) I encountered the problem where a non-MBC ROM needs to not have "#pragma bank=" in the mod2gbt output. I didn't see an option to suppress it so I've added it here.

When the toolchain is set for non-MBC mode it treats "_CODE" as 32K instead of two 16K regions (_CODE and CODE_1) . If you try to use any bank specifier (such as bank=1) then you get the error: "ERROR: address overflow (addr c51b >= 8000)"

This has also been discussed in more detail here: https://gbdev.gg8.se/forums/viewtopic.php?id=466

The patch:

• Add support for unbanked mode. When bank is set to zero ("0") it will suppress the "#pragma bank=" output. There is no loss of functionality (as far as I know) since "#pragma bank=0" is not supported anyway- you specify that by the absence of the bank #pragma.

• Fix a missing space after a "const " specifier

[AntonioND]

Looks good to me. However, can you split this into two patches?

• Support for unbanked output in legacy converter
• Fix missing space after const specifier

[bbbbbr]

Sure, this better?

[AntonioND]

Yes, thanks!