system_info_generated: Print boilerplate for API description

system_info_generated: Print boilerplate for API description

The generated header now contains boilerplate describing the generated
getter or enum declaration. For this purpose, the SystemInfoGenerator
now understands the below scheme of model-config.xml.

 .//key[@since_tizen]
   : The attribute 'since_tizen' appear as @since_tizen at the
     getter description

 .//enumerator[@since_tizen]
   : The attribute 'since_tizen' appear as @since_tizen at the
    enumerator description

 .//enumerator[@desc]
   : The attribute 'desc' appear as a string following @since_tizen
     of enumerator

Change-Id: I0769e9f0dabda151e930b5374420d36df785191a
Signed-off-by: Youngjae Cho <y0.cho@samsung.com>