Use kernel_configure_variable in append for rpi

This changeset simplifies the configuration of the rpi kernel
by using kernel_configure_variable in do_configure_append.
Due to an upstream bug, we need to repeat the final configuration call.

We also enable BLK_DEV_RAM, BLK_DEV_NBD and SMACK.

For this we also define an OVERRIDE for netboot.

v2: add conditional appends for netboot, sota, smack by using overrides.

Change-Id: I2a4a8fae6989246d715fc2fc71bb4c251f423bdb
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>