added "gen.<boardname>" target to Makefile.common

Change-Id: Icbfe746c6d136092b004bc9cacbe5b8df9854536
added implicit rules make {gen,build,run}.{boardname}[.gdb]

Change-Id: I1859cad59acbe9dfb77437d43acc321d0797d6a1
make target gen:: -> gentarget::

Change-Id: I0ee2040319843269bf12d0104cdeec4e5aa4d332
added implicit make rule for .gdb suffix to set XBSV_DEBUG=1, e.g. echo.zedrun.gdb

Change-Id: Iefbc3be69df6a96626e3c372d1bd12debb8cdf07