19881: drivers/stmpe811: fix initialization if callback function parameter is NULL r=benpicco a=gschorcht
### Contribution description
This PR fixes the `stmpe811` driver initialization if the callback function parameter `cb` is `NULL. This might be the case for example if the application uses the touch device in polling mode.
If the interrupt pin is initialized if the callback function parameter `cb` is `NULL`, the driver crashes the first time an interrupt is triggered. Therefore, the INT pin must be initialized only if also the callback function parameter `cb` is not `NULL`.
To be able to test the polling mode, this PR also includes a change of the `tests/drivers/stmpe811` application which introduces the environment variables `STMPE811_POLLING_MODE` `STMPE811_POLLING_PERIOD` and in the makefile.
### Testing procedure
1. Use a `stm32f429i-disc1` board and test it in polling mode:
```
STMPE811_POLLING_MODE=1 BOARD=stm32f429i-disc1 make -C tests/drivers/stmpe811 flash term
```
It should work as expected.
```
main(): This is RIOT! (Version: 2023.10-devel-119-g26e7a-drivers/stmpe811_fix_cb_null)
STMPE811 test application
+------------Initializing------------+
Initialization successful
Pressed!
X: 113, Y:135
X: 113, Y:135
X: 113, Y:136
Released!
```
2. Checkout master branch and cerry-pick commit 691a5e6308426ddc685e5a2c297238529211c258. The test application `tests/drivers/stmpe811` will crash once a touch event occur:
```
main(): This is RIOT! (Version: 2023.10-devel-117-g91441)
STMPE811 test application
+------------Initializing------------+
Initialization successful
Stack pointer corrupted, reset to top of stack
FSR/FAR:
CFSR: 0x00020000
HFSR: 0x40000000
DFSR: 0x00000008
AFSR: 0x00000000
Misc
EXC_RET: 0xfffffff1
*** RIOT kernel panic:
HARD FAULT HANDLER
```
### Issues/PRs references
19892: pkg/tinydtls: allow to set buffer size from application again r=benpicco a=leandrolanzieri
### Contribution description
Currently the buffer size on tinydtls is set in its Makefile whenever `gcoap` module is present. This limits the ability of the user to override the value. This adds a pre-check of the `CFLAGS` to see if it was set before.
### Testing procedure
Try setting `CFLAGS += -DDTLS_MAX_BUF=<some_value>` on `examples/gcoap_dtls`, you should be able to override the default value without errors.
### Issues/PRs references
Reported in #19838
Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
Co-authored-by: Leandro Lanzieri <leandro.lanzieri@haw-hamburg.de>
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