uboot-config.bbclass: Raise error for bad key

If an invalid key is used, the class ignores the error, with an
indeterminate result. In my case, the problem surfaced in do_deploy:

```
| cp: cannot stat '/.../build/tmp/work/imx6qdlsabresd-fsl-linux-gnueabi/u-boot-imx-mfgtool/2022.04-r0/deploy-u-boot-imx-mfgtool/u-boot.imx': No such file or
| directory
```

The root cause of this was that the uboot config key did not match
a valid option. With the fix, the error is caught by the class:

```
ERROR: Nothing PROVIDES 'u-boot-imx-mfgtool'
u-boot-imx-mfgtool was skipped: The selected UBOOT_CONFIG key ['mfgtool'] has no match in dict_keys(['sd-fslc', 'sd-imx', 'sd-optee-imx', 'sata-imx', 'mfgtool-imx']).
```

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>