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
diff --git a/src/meson.build b/src/meson.build
index 6a03a3e..64a2b86 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -14,6 +14,11 @@
     'templates/event.mako.cpp',
 )
 
+yamldir = get_option('YAML_PATH')
+if yamldir == ''
+    yamldir = meson.project_source_root() + '/src/example'
+endif
+
 generated_hpp = custom_target(
     'generated.hpp',
     command: [
@@ -21,7 +26,7 @@
         meson.project_source_root() + '/src/pdmgen.py',
         '-t', 'generated.mako.hpp',
         '-p', meson.project_source_root() + '/src/templates',
-        '-d', meson.project_source_root() + '/' + get_option('YAML_PATH'),
+        '-d', yamldir,
         '-o', meson.current_build_dir() + '/generated.hpp',
         'generate-cpp'
     ],