19365: bootloaders: fix bootloader button logic r=benpicco a=dylad
### Contribution description
In lastest master, the `BTN_BOOTLOADER_INVERTED` logic doesn't work as expected.
This PR fixes the underlying logic by replacing the `BTN_BOOTLOADER_INVERTED` macro definition by a runtime function.
In fact the current code:
```
#ifndef BTN_BOOTLOADER_INVERTED
#if (BTN0_MODE == GPIO_IN_PD)
#define BTN_BOOTLOADER_INVERTED false
#else
#define BTN_BOOTLOADER_INVERTED true
#endif
#endif
```
cannot work because both `BTN0_MODE` and `GPIO_IN_PD` are not known by the precompiler as they are enum values defined at cpu level.
Thus, replaces it by a runtime function in our bootloader applications.
I've also add `GPIO_OD_PU` along side `GPIO_IN_PU` and add a new define (which can be override at board level or app level) in case an external pullup is used.
### Testing procedure
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`
With master, the application will not start.
With this PR, the application will start after flashing.
### Issues/PRs references
Fixes#19364
19366: nanocoap_sock: don't include token in empty ACK response r=benpicco a=benpicco
19367: cord: bump reference from draft to rfc r=benpicco a=bergzand
### Contribution description
The draft is an RFC, this bumps the "see also" in the docs to the rfc.
### Testing procedure
Check that the correct RFC is linked in the docs.
### Issues/PRs references
None
Co-authored-by: Dylan Laduranty <dylan.laduranty@mesotic.com>
Co-authored-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Co-authored-by: Koen Zandberg <koen@bergzand.net>
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