How to create a non-901226-01 basic.bin ?

[hartkopp]

The current build process just creates a basic.bin for the 901226-01 - and sets the checksum accordingly.

What is the reason for the switch between 'old' and 'new' check in the Makefile?

Is it to create a 64GS ROM with other fill bytes according to
http://www.zimmers.net/anonftp/pub/cbm/firmware/computers/c64/index.html

basic.901226-01.bin

Commodore 64 BASIC V2. The first and essentially only revision,
located at $A000-$BFFF. In the 64GS system firmware, the 30 unused
bytes at $BF53-$BF70 are filled with $00 instead of $AA. In the
Commodore 65 firmware, these bytes are filled with $FF.