ewaol-distro: Use Cassini distro user variables

cassini-distro: Use OVERRIDES cassini in bbappend

Cassini:

The meta-cassini-distro contains bbappend files which causes yocto
layer-check test to fail if not made conditional. Therefore, enable
bbappend files only when `cassini` is available in `OVERRIDES`.

The package group cassini is removed as this was causing significant
overhead in EWAOL guest vm image configuration. The `k3s-server`
package. All the required packages for Cassini are part of
`meta-cassini-distro/classes/cassini-image.bbclass`.

EWAOL:

Inherit `cassini_security` distro feature.

The cassini distro defines following user variables:

CASSINI_USER_ACCOUNT
CASSINI_ADMIN_ACCOUNT
CASSINI_TEST_ACCOUNT
CASSINI_ADMIN_GROUP
CASSINI_ADMIN_GROUP_OPTIONS

and creates those user accounts and groups in
meta-cassini/meta-cassini-distro/classes/cassini-image.bbclass.
Therefore, use those for creating EWAOL user variables.

The patch 0001-xen-move-x86-configs-into-a-separate-file.patch moves
xen-x86 config to a separate file.

Reference:
https://gerrit.oss.arm.com/gitweb?p=ais/ewaol/meta-ewaol.git;a=commit;h=489b48b6325a2dcd9683686fb14d07f4fd29f934



Issue-Id: OSSEMBEDA-1787

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>
Change-Id: I846b83a3b778f6ff231165b3bd2dffd8f881b7b4