[build] Allow USB to be set to 0

[tyomitch]

USB_AVAILABLE was conditionally defined in supervisor.mk,
but never actually used to #ifdef USB-related code.

Loosely related to #4546

[dhalbert]

I'm confused about USB vs USB_AVAILABLE. Did you find who defines USB? Could you merge the uses of these to into a single toggle, like CIRCUITPY_USB_AVAILABLE or CIRCUITPY_USB?

There's an #if USB_AVAILABLE (not #ifdef) in litex/supervisor/internal_flash.c.

It would also be good to not use #ifdef USB_AVAILABLE, but instead always define it as 0 or 1, and use #if USB_AVAILABLE (or some new name).

[tyomitch]

I'm confused about USB vs USB_AVAILABLE.

USB is a makefile variable, USB_AVAILABLE is a C definition: both were added in 9d91111, but apparently both stayed unused -- except in litex/supervisor/internal_flash.c since 786e79e, which would never work as intended because USB_AVAILABLE is either empty or undefined.

Did you find who defines USB?

None of the in-tree targets; a target would inevitably fail to compile if it did.

microbit_v2, when/if added, should become the first one.

[dhalbert]

USB is a makefile variable, USB_AVAILABLE is a C definition: both were added in 9d91111, but apparently both stayed unused -- except in litex/supervisor/internal_flash.c since 786e79e, which would never work as intended because USB_AVAILABLE is either empty or undefined.

Could you unify them, so it's simpler? Thanks.

[tannewt]

One tweak. Otherwise it looks good. Thanks!

[dhalbert]

Thank you for cleaning this up!

changes made