Generalize EULA functions and apply to imx6qsabrelite

This change generalizes the *_setup_EULA*.sh scripts to work with custom layouts.
The 50_setup.sh of the machine template just needs to call
find_and_ack_eula <layerfolder> [<EULA_FILENAME_relative_to_layerfolder>]

The EULA_FILENAME is optional.

Also an optional EULA_FLAG_NAME can be exported which will be used
for the layer.conf.
If no EULA_FLAG_NAME is set, we'll default to the current
ACCEPT_EULA_$MACHINE.

Change-Id: I03a4551e1cf088dce0b13aa5e38f606d67e0fb29
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>