contrib: u-boot: testpy: Add support for out-of-tree U-Boot builds

[Rahix]

Add a uboot_builddir parameter which can be used to specify the location of an out-of-tree U-Boot build when such an environment is used.

I do not have an environment to test this change right now. If anyone can report whether this works as intended, I'll merge the change :)

Closes #108.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (81b8d03) 62.47% compared to head (b603bb5) 62.37%.

Files	Patch %	Lines
tbot_contrib/uboot/_testpy.py	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #109 +/- ## ========================================== - Coverage 62.47% 62.37% -0.11% ========================================== Files 53 53 Lines 3731 3734 +3 ========================================== - Hits 2331 2329 -2 - Misses 1400 1405 +5 ```

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