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>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index ddef3d0..ef79fc3 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -2352,6 +2352,7 @@
 source "board/isee/igep003x/Kconfig"
 source "board/kontron/sl28/Kconfig"
 source "board/myir/mys_6ulx/Kconfig"
+source "board/siemens/common/Kconfig"
 source "board/seeed/npi_imx6ull/Kconfig"
 source "board/socionext/developerbox/Kconfig"
 source "board/st/stv0991/Kconfig"