feat: support building with user prebuilt rootfs

[prodeveloper0]

Support building with user prebuilt rootfs by setting .BoadConfig.mk CONFIG_USE_PREBUILT_ROOTFS=y

[prodeveloper0]

It can reduce build time of firmware by ignoring to build rootfs and its components. just compile kernel and uboot with prebuilt rootfs!

[luckfox-eng33]

It may cause the media library related to rk to be unusable.
And the absence of corresponding prebuilt rootfs ,may cause more confusion for some novices.

I might be inclined not to merge into the master branch.
But thank you for wanting to contribute to our community.

[prodeveloper0]

@luckfox-eng33 yes. this is advanced user feature who does not want to use OEM toolkit.
if you worried about claim from some novices, need to notify this is advanced user feature and it caused some errors or fails how you build custom rootfs.

[luckfox-eng33]

「bug report」RV1106_Luckfox_Pico_Ultra_W「错误报告」RV1106_Luckfox_Pico_Ultra_W Why is rootfs only 55m in capacity, it should be 7G in capacity?为什么rootfs容量只有55m，应该是7G容量？

「bug report」RV1106_Luckfox_Pico_Ultra_W Why is rootfs only 55m in capacity, it should be 7G in capacity?

1. It seems that the root file system does not have a normally mounted partition. It can be restored through resize2fs. For specific operations, please refer to the related operations of buildroot.
2. If you need to return a product, you can contact your dealer and do not need to declare it here

[luckfox-eng33]

@luckfox-eng33 yes. this is advanced user feature who does not want to use OEM toolkit. if you worried about claim from some novices, need to notify this is advanced user feature and it caused some errors or fails how you build custom rootfs.

We are conducting a careful evaluation and our FAE team is exhausted by too many questions.
Maybe we can create a new branch specifically for users who need to build rootfs separately, instead of merging it into the main line, what do you think?

[prodeveloper0]

@luckfox-eng33 it's okay. if you were worried about exhausting your team to answer to the questions, it's good to separate branch technical supporting and non-technical supporing branch.