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]

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%.

❗ 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 ⚠️

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.