keymile boards: support of boardId / hwkey lists

In order to support boardId / hwkey lists, the u-boot default
environment has been updated: Added a script checkboardidlist
which checks the list of boardId / hwkey if the boadrId / hwkey
of the IVM is included in that list. This feature is used if you
got different HW variants but you only want to create one boot
package. E.g. supx5 board series.

Signed-off-by: Thomas Herzmann <thomas.herzmann@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Heiko Schocher <hs@denx.de>