Fix getting absolute path to YAML_PATH

When we extend the `YAML_PATH` in bb or bbappend,
the meson.project_source_root should not get the absolute path in
meson.build, this will produce double paths and cause the build to
fail.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I6de51da8b9fda4d00a727211edd3448802f84a43
diff --git a/meson_options.txt b/meson_options.txt
index c124f67..fce4972 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -6,6 +6,6 @@
 
 option(
     'YAML_PATH', type: 'string',
-    value: 'src/example',
+    value: '',
     description: 'The path to the yaml config files.'
 )
\ No newline at end of file