in Makefile.common, PINS defaults to Std

Change-Id: I6d276364f19f1de254078985dc205206892fd4d2
documented the variables in echo/Makefile

Change-Id: I689ceca33c829f6146aefd176eb872d1cb42421b
some documentation of design structure (Makefiles)

Change-Id: Ie4099663a7e874d2415bcb856cd709437688dab8