19128: boards: common: stdio_cdc_acm: let tests wait a bit for serial port r=aabadie a=miri64



19133: makefiles/utils/strings.mk: Fix version_is_greater_or_equal r=maribu a=maribu

### Contribution description

The Makefile function `version_is_greater_or_equal` is used to check if a version of GNU Make is at least the required one. However, it has the built-in assumption the version numbers have to format x.y.z, but Alpine Linux currently ships GNU Make 4.4. This results in `$(call _pad_number,3,)` which runs `printf '$03d' ''` in the shell, which is not valid.

This fixes the issue by making `_pad_number` more robust by fall back to printing `0` with the given padding, if the number given to print is empty.

### Testing procedure

Append

```Makefile

$(info A=$(call version_is_greater_or_equal,4.2.0,4.2.0))
$(info B=$(call version_is_greater_or_equal,4.2,4.2.0))
$(info C=$(call version_is_greater_or_equal,4.1,4.2.0))
$(info D=$(call version_is_greater_or_equal,4.1.9,4.2.0))
$(info E=$(call version_is_greater_or_equal,5.1.9,4.2.0))
$(info F=$(call version_is_greater_or_equal,5.0.0,4.2.0))
$(info G=$(call version_is_greater_or_equal,4.2.1,4.2.0))
$(info H=$(call version_is_greater_or_equal,4.3.1,4.2.0))
```

e.g. to `makefiles/utils/strings.mk`, build something and observe the info output.

This yields

```
A=1
B=1
C=
D=
E=1
F=1
G=1
H=1
```

for me and does not complain about invalid `printf` invocations.

### Issues/PRs references

None

Co-authored-by: Martine Lenders <m.lenders@fu-berlin.de>
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>