test_build.sh: Move build test procedure to repository

Instead of hard coding the test procedure on qa.coreboot.org, allow
running a script in the repo instead. The server is already adapted
to do that, so once there's a test_build.sh file in the toplevel
directory, it's run in place of the default operation.

The content of this change mirrors the default operation exactly so
should serve as a good starting point.

The script is executed in an encapsulate[0] context with the workspace,
/tmp and $HOME/.ccache writable, everything else read-only and
network disabled.

It should return 0 on success, anything else on failure, as is normal
for UNIX processes.

[0] https://review.coreboot.org/cgit/encapsulate.git

(Backported minus the Meson support)

Change-Id: I37a8e925d1b283c3b8f87cb3d0f1ed8920f2cf95
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/46894
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/62614
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

test_build.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+set -e
+
+make CONFIG_EVERYTHING=yes WARNERROR=yes