km/common: add test sw starting variable

The test SW is started when the test_bank variable is set.

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