bsp: k230: improve cleanup

Previously, `scons -c` did not clean up the build directory
and the automatically generated link.lds.generated.

This is fixed now.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

bsp/k230/SConstruct

@@ -56,3 +56,6 @@ generate_ldscript('link.lds', 'link.lds.generated')
 
 # make a building
 DoBuilding(TARGET, objs)
+
+Clean(TARGET, 'link.lds.generated')
+Clean(TARGET, 'build')