boards: Remove CONFIG_OSC_EXTERNAL defconfigs from non-kinetis boards

This config is only valid for kinetis boards. Fixes the Kconfig warning:

warning: the choice symbol OSC_EXTERNAL (defined at
arch/arm/soc/nxp_kinetis/Kconfig:53) was selected (set =y), but no
symbol ended up as the choice selection.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>