Generalize overrides subsystem for NXP and Mainline support

Generalize overrides subsystem for NXP and Mainline support

Essentially, we extend the overrides to a generic-bsp, nxp-bsp, and
mainline-bsp.

So, for example, the mx8mq override is split into:

- imx-generic-bsp: compatible with every i.MX SoC and both BSP variants
- imx-nxp-bsp: compatible with every i.MX SoC but specific to NXP BSP
- imx-mainline-bsp: compatible with every i.MX SoC but specific to Mainline BSP

- mx8-generic-bsp: compatible with every i.MX8 SoC and both BSP variants
- mx8-nxp-bsp: compatible with every i.MX8 SoC but specific to NXP BSP
- mx8-mainline-bsp: compatible with every i.MX8 SoC but specific to Mainline BSP

- mx8m-generic-bsp: compatible with every i.MX8M SoC and both BSP variants
- mx8m-nxp-bsp: compatible with every i.MX8M SoC but specific to NXP BSP
- mx8m-mainline-bsp: compatible with every i.MX8M SoC but specific to Mainline BSP

- mx8mq-generic-bsp: compatible with every i.MX8MQ SoC and both BSP variants
- mx8mq-nxp-bsp: compatible with every i.MX8MQ SoC8 but specific to NXP BSP
- mx8mq-mainline-bsp: compatible with every i.MX8MQ SoC but specific to Mainline BSP

The extender mechanism is responsible for extending the override list to
include the generic overrides. We can then use the three different
variants to handle the metadata correctly.

Generically speaking, the conversion mainly was automated (with a lot of
back and forth until getting it right).

To convert an existing layer, the following script can be used:

```sh
  git ls-files classes recipes-* \
      | xargs sed -i \
              -e 's,:\(mx[6-8]\w*\),:\1-nxp-bsp,g' \
              -e 's,(\(mx[6-8]\w*\)),(\1-nxp-bsp),g' \
              -e 's,\(mx[6-8]\w*\)|,\1-nxp-bsp|,g' \
              -e 's,|\(mx[6-8]\w*\)),|\1-nxp-bsp),g' \
              \
              -e 's,:\(mx[5s]\w*\),:\1-generic-bsp,g' \
              -e 's,(\(mx[5s]\w*\)),(\1-generic-bsp),g' \
              -e 's,\(mx[5s]\w*\)|,\1-generic-bsp|,g' \
              -e 's,|\(mx[5s]\w*\)),|\1-generic-bsp),g' \
              \
              -e 's,:\(vf\w*\),:\1-generic-bsp,g' \
              -e 's,:\(vf[56]0\w*\),:\1-generic-bsp,g' \
              -e 's,\(vf\w*\)|,\1-generic-bsp|,g' \
              -e 's,|\(vf\w*\)),|\1-generic-bsp),g' \
              -e 's,\(vf[56]0\w*\)|,\1-generic-bsp|,g' \
              -e 's,|\(vf[56]0\w*\)),|\1-generic-bsp),g' \
              \
              -e 's,:\(imx\) ,:\1-nxp-bsp ,g' \
              -e 's,(\(imx\)),(\1-nxp-bsp),g' \
              -e 's,\(imx\)|,\1-nxp-bsp|,g' \
              -e 's,|\(imx\)),|\1-nxp-bsp),g'

  for d in $(find -type d | egrep '/mx[6-8]w*'); do
      git mv $d $d-nxp-bsp
  done

  for d in $(find -type d | egrep '/imx$'); do
      git mv $d $d-nxp-bsp
  done

  for d in $(find -type d | egrep '/mx[5s]w*'); do
      git mv $d $d-generic-bsp
  done
```

Fixes: #791.
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>