small build system and config files reorganization

CONFIG.md

@@ -3,9 +3,11 @@
 
 It's not possible to provide ROM builds that suit everyone needs - therefore configuration files were introduced, few predefined configurations are provided with sane defaults:
 
+* [`c64/,,config_custom.s`](c64/,,config_custom.s)
 * [`c64/,,config_generic.s`](c64/,,config_generic.s)
 * [`c64/,,config_mega65.s`](c64/,,config_mega65.s)
 * [`c64/,,config_ultimate64.s`](c64/,,config_ultimate64.s)
+* [`c64/,,config_testing.s`](c64/,,config_testing.s)
 
 Just edit them and recompile the project. To enable particular option - uncomment it by removing leading `//` from before `#define` directive (as you probably guessed, commenting out disables it). Some options are followed by constants - you can change them too to further fine-tune the build.
 
@@ -27,6 +29,10 @@ Since Open ROMs is highly modular, it should be possible to add other platforms
 
 Select if the ROM is going to be used exclusively on the specific motherboard. It prevents from enabling options not having sense, skips initialization of C128-only registers, etc.
 
+### `CONFIG_BRAND_CUSTOM`
+
+Intended for custom builds, which are not to be redistributed. Allows to configure branding with `CONFIG_CUSTOM_BRAND` varaible
+
 ### `CONFIG_BRAND_GENERIC`
 
 If you don't know which variant to choose - select this one.