ewaol-distro/images: Add sdk image flavour

This patch adds 'ewaol-sdk' DISTRO_FEATURE to ease image customization.
It contains set of libraries, headers and packages that are useful during
software development on target board.
'ewaol-sdk' is inherited by default in EWAOL SDK images:
 * ewaol-image-docker-sdk
 * ewaol-image-podman-sdk

ewaol-sdk enables:
 * Packages:
   - gator-daemon
   - kernel-devsrc
   - kernel-modules
 * IMAGE_FEATURES:
   - package-management
   - dev-pkgs
   - tools-debug
   - tools-sdk
   - tools-profile
   - debug-tweaks
   - ssh-server-openssh

Issue-ID: SCM-2394
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Change-Id: I22ea4ead8711671af6d9bc360f45748031720eca