The `$(bin2o)` line in the makefile is a GNU-specific syntax, it calls a
"macro" defined in the DevkitPro framework (a makefile that is included
in the `$(DEVKITARM/gba_rules)` makefile, which itself is included in
this makefile)

The macro content is the new lines that replaces the call to
`bin2o` in this commit.

However, it seems macros are buggy in GNU make, as the `$(@)` variable
(containing the "target" of the rule, or the `%.o` in the rule defined
in this repo's makefile) is not the same inside the macro than inside
the rule. It resulted in the output files being called
`bullet_til.bin.o` instead of `bullet_til.o`, which resulted in
compilation failing.

Inlining the macro reverted `$(@)` to resolve to `%.o` as it should,
rather than `%.bin.o`, which solved the problem.