19618: cpu/stm32: fix riotboot settings for L4 and WB r=benpicco a=gschorcht
### Contribution description
This PR fixes the `riotboot` configuration for L4 and WB.
The family is not called `stm32l4` or `stm32wb` but `l4` and `wb`. That is, the `riotboot` configuration didn't work at all. Furthermore, a minimum `RIOTBOOT_LEN` of `0x2000` is required for L4.
Found when investigating the compilation errors for `bootloaders/riotboot_serial` in PR #19576.
### Testing procedure
1. Green CI.
2. Use the following commands:
```
BOARD=nucleo-l496zg make -C tests/riotboot info-debug-variable-RIOTBOOT_HDR_LEN
BOARD=p-nucleo-wb55 make -C tests/riotboot info-debug-variable-RIOTBOOT_HDR_LEN
```
In master these commands give
```
0x400
```
With this PR these commands give
```
0x200
```
as expected.
3. Use the following commands:
```
BOARD=nucleo-l496zg make -C tests/riotboot info-debug-variable-RIOTBOOT_LEN
BOARD=p-nucleo-wb55 make -C tests/riotboot info-debug-variable-RIOTBOOT_LEN
```
In master these commands give
```
0x1000
```
With this PR these commands give
```
0x2000
```
as expected.
### Issues/PRs references
19639: tests/net/gnrc_mac_timeout: add automated test r=aabadie a=aabadie
19644: gnrc_ipv6_nib: include RIO with all subnets in downstream RA r=benpicco a=benpicco
19649: gnrc_sixlowpan_iphc: prefix bits outside context must be zero r=benpicco a=benpicco
19656: gnrc/ipv6_auto_subnets: allow to configure minimal prefix length r=benpicco a=benpicco
Co-authored-by: Gunar Schorcht <gunar@schorcht.net>
Co-authored-by: Alexandre Abadie <alexandre.abadie@inria.fr>
Co-authored-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>
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