plugins: kbuild: read configs from source directory instead

[jessesung]

Configs should be read from source directory instead of TOPDIR, in order
to handle a remote git repo properly.

Signed-off-by: Wen-chien Jesse Sung jesse.sung@canonical.com

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• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh unit?

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[piso77]

You can incur in a problem if, for example, you have a snapcraft.yaml that points to a remote git tree and branch:

snapcraft.yaml:
...
parts:
kernel:
plugin: kernel
source: https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/xenial
source-branch: raspi2
source-type: git
kconfigflavour: raspi2
...

while you current working directory contains the debian structure with the config files - the kernel plugin won't take in consiresation the remote debian dir, and use the local one:

Traceback (most recent call last):
File "/home/ubuntu/snapcraft/snapcraft/plugins/kbuild.py", line 158, in assemble_ubuntu_config
with open(config_part_path) as config_part:
FileNotFoundError: [Errno 2] No such file or directory: 'debian.master/config/armhf/config.flavour.raspi2'

this patch will fixes that - i replicated it locally, and it does what it says.

[sergiusens]

thanks for contributing @jessesung
Do you mind fixing the static tests or if I go ahead and do so myself and push to this PR?

[codecov-io]

Codecov Report

Merging #2202 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2202      +/-   ##
==========================================
+ Coverage   91.17%   91.19%   +0.02%     
==========================================
  Files         202      202              
  Lines       12987    12987              
  Branches     1921     1921              
==========================================
+ Hits        11841    11844       +3     
+ Misses        777      776       -1     
+ Partials      369      367       -2

Impacted Files	Coverage Δ
snapcraft/plugins/kbuild.py	88.57% <100%> (ø)	⬆️
snapcraft/internal/elf.py	83.17% <0%> (+0.32%)	⬆️
snapcraft/internal/pluginhandler/_plugin_loader.py	93.75% <0%> (+1.78%)	⬆️

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