meson: Fix BUSNAME argument to pimgen.py

Meson does not support querying environment variables. The old automake
build systems used the $BUSNAME environment variable to compile PIM with
the busname it needs to request.

Since moving to meson, this does not work as intended and was causing
pimgen to generate bad C++ code.

BUSNAME is now setup as a configuration data. This commit queries the
configuration data for the BUSNAME and passes it on as an argument to
pimgen.py.

Tested:

Tested by bitbaking a witherspoon image. PIM now compiles cleanly.

Change-Id: I567841231c8f295442e6d372d2a758737fcf70c8
Signed-off-by: Santosh Puranik <santosh.puranik@gmail.com>
diff --git a/meson.build b/meson.build
index d62d0ff..b3fe39c 100644
--- a/meson.build
+++ b/meson.build
@@ -83,7 +83,7 @@
         '-i', ifacesdir,
         '-d', get_option('YAML_PATH'),
         '-o', meson.current_build_dir(),
-        '-b', '$BUSNAME',
+        '-b', conf_data.get_unquoted('BUSNAME'),
         'generate-cpp'
     ],
     output : 'generated.cpp')
@@ -96,7 +96,7 @@
         '-i', ifacesdir,
         '-d', get_option('YAML_PATH'),
         '-o', meson.current_build_dir(),
-        '-b', '$BUSNAME',
+        '-b', conf_data.get_unquoted('BUSNAME'),
         'generate-serialization'
     ],
     output : 'gen_serialization.hpp')