siemens: board: clean up products folders vs common

The common folder was initialially created for the common parts of the
products based on draco-am355x board family. These are the product lines
'pxm2', 'rut' and the base line named 'draco'!

Adding the new capricorn-imx8 board family, common was enhanced without
cleanup.
- rename 'common/board.c' to 'common/board_am335x.c'
- add 'common/board_am335x.h' for export to the product lines

Reviewed-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Signed-off-by: Enrico Leto <enrico.leto@siemens.com>
diff --git a/board/siemens/rut/MAINTAINERS b/board/siemens/rut/MAINTAINERS
index 1e92710..4d8e256 100644
--- a/board/siemens/rut/MAINTAINERS
+++ b/board/siemens/rut/MAINTAINERS
@@ -1,5 +1,5 @@
 RUT BOARD
-M:	Samuel Egli <samuel.egli@siemens.com>
+M:	Enrico Leto <enrico.leto@siemens.com>
 S:	Maintained
 F:	board/siemens/rut/
 F:	include/configs/rut.h