getty: Create specific getty config + cleanups

This commit introduces a specific BR2_TARGET_GENERIC_GETTY configuration flag.
This eliminates the need for checking if BR2_TARGET_GENERIC_GETTY_PORT is an
empty string or not. It also allows hiding various getty options when getty
isn't enabled.

Signed-off-by: Assaf Inbal <shmuelzon@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>