siemens: Move CONFIG_FACTORYSET to Kconfig

Introduce board/siemens/common/Kconfig and have it hold FACTORYSET to
start with. Use select for this on the boards that need it.

Cc: Anatolij Gustschin <agust@denx.de>
Cc: Samuel Egli <samuel.egli@siemens.com>
Signed-off-by: Tom Rini <trini@konsulko.com>