build: allow to define additional configs

[a-gave]

This introduces the possibility of passing additional configs to the request.

Only the few not related to kernel/packages, as they are not recompiled by the imagebuilder, would make sense. For example, to request a specific name/number for a firmware image:

CONFIG_VERSION_DIST=MyRouterOS
CONFIG_VERSION_NUMBER=1.6

Or to request a format other than the default format for the output firmware image, for example:

# CONFIG_TARGET_ROOTFS_SQUASHFS is not set
CONFIG_TARGET_ROOTFS_JFFS2=y

[aparcar]

Please fix the failing test.

[codecov[bot]]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (6f634f8) 80.94% compared to head (60b45b9) 86.70%.

:exclamation: Current head 60b45b9 differs from pull request most recent head 88f632b. Consider uploading reports for the commit 88f632b to get more accurate results

Files	Patch %	Lines
asu/build.py	18.18%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #728 +/- ## ========================================== + Coverage 80.94% 86.70% +5.75% ========================================== Files 15 15 Lines 976 985 +9 ========================================== + Hits 790 854 +64 + Misses 186 131 -55 ```

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