use relative layer dirs to make build relocatable

This patch replaces the absolute paths that are injected into BBLAYERS
by relative ones. These are relative to TOPDIR.
By that, the whole build directory becomes relocatable.

This is of value when using a shared sstate cache and build machines
with varying build locations (e.g. gitlab-ci runners).

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/libcmds.py

@@ -260,6 +260,16 @@ class WriteBBConfig(Command):
         return 'write_bbconfig'
 
     def execute(self, ctx):
+        def _get_layer_path_under_topdir(ctx, layer):
+            """
+                Returns a path relative to ${TOPDIR}.
+                TOPDIR is a BB variable pointing to the build directory.
+                It is not expanded by KAS, hence we avoid
+                absolute paths pointing into the build host.
+            """
+            relpath = os.path.relpath(layer, ctx.build_dir)
+            return '${TOPDIR}/' + relpath
+
         def _write_bblayers_conf(ctx):
             filename = ctx.build_dir + '/conf/bblayers.conf'
             if not os.path.isdir(os.path.dirname(filename)):
@@ -268,7 +278,8 @@ class WriteBBConfig(Command):
                 fds.write(ctx.config.get_bblayers_conf_header())
                 fds.write('BBLAYERS ?= " \\\n    ')
                 fds.write(' \\\n    '.join(
-                    sorted(layer for repo in ctx.config.get_repos()
+                    sorted(_get_layer_path_under_topdir(ctx, layer)
+                           for repo in ctx.config.get_repos()
                            for layer in repo.layers)))
                 fds.write('"\n')
                 fds.write('BBPATH ?= "${TOPDIR}"\n')