We need to boot any installer of distribution independent on DISTRI
and VERSION variables. New variable IPXE_BOOT_FIXED indicates that we
want to boot fixed version. It will boot by default SLE 15-SP6, but it
is possible to override it by IPXE_BOOT_FIXED_DISTRI and
IPXE_BOOT_FIXED_VERSION variables.

Enhancement poo#167021: SL Micro doesn't have installer like openSUSE
or SLE. Only raw images are supported. If we want to install SL Micro
to bare metal, we need to directly copy image to destination, this is
documented and supported way. The idea behind this extension, is to
use existing openQA boot modules for particular backends and boot SLE
installer, use installation console to mount nfs share from openQA and
deploy image to target disk. We need to modify grub configuration
before first boot, to be able to see menu in hardware consoles. root
password is configured by ignition, which has own separate parition on
the same disk. There are supported following backends and
architectures hmc/ppc64le, ipmi/x86_64/aarch64. Variables MIRROR_HTTP
and REPO_0 are needed to properly boot installer on hmc
backend. Variable IPXE_BOOT_FIXED must be defined for ipmi. There can
be used optional variables IPXE_BOOT_FIXED_DISTRI and
IPXE_BOOT_FIXED_VERSION to boot specific installer version. It is
possible to specify target disk on all backends  by variable
MICRO_INSTALL_IMAGE_TARGET_DEVICE, the default is /dev/sda.