Makefile: add targets for Debug builds

[tobiasvl]

Would it perhaps be useful to add debug targets to the Makefile, so they're easy to build?

It could perhaps have the most useful subset of debug functionality enabled, instead of everything; for example, only enable debug modes 1 and 3 (2 is less useful).

Not sure about giving the debug save file a name that starts with a single space (" DBUG"?) so Marin can be used for text debugging (mostly useful for translators). If so, the starting room should perhaps be made into a variable and configured to be different from the Rooster statue, otherwise the file can't be used for anything but text debugging.

https://tcrf.net/The_Legend_of_Zelda:_Link%27s_Awakening_(Game_Boy)#Debug_Utilities

[kemenaran]

Sure, a debug target could be a good idea.

It is already possible to build a debug ROM by defining the ENABLE_DEBUG_1 constant, like this:

$ make ASFLAGS='--define ENABLE_DEBUG_1'

But a make debug could be nice (even if it only builds the EN 1.0 rom, that's already something)