Description

The logic to define the value of BTN_BOOTLOADER_INVERTED doesn't work as expected.
This lead to issue in bootloader/riotboot_dfu and bootloader/riotboot_tinyusb_dfu.
For instance, on my SAML21-XPRO, I can successfully flash any application using dfu-util after flashing bootloader/riotboot_dfu

Steps to reproduce the issue

Flash the riotboot_dfu bootloader:
make BOARD=saml21-xpro -C bootloaders/riotboot_dfu flash
Then, flash any test app:
PROGRAMMER=dfu-util USEMODULE=usbus_dfu make BOARD=saml21-xpro -C tests/shell riotboot/flash-slot0

Expected results

Application should start after flashing.

Actual results

Flash succeed, but application doesn't start.
a quick look at lsusb shows that we're stuck in the bootloader (with VID/PID: 1209:7d02)

I'll provide a PR to fix that right away.