Add KAS_PREMIRRORS support

Analogously to bitbake's PREMIRRORS, this allows to define alternative
sources for repo URLs specified in kas files.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Daniel Wagner <daniel.wagner@siemens.com>

kas-docker

@@ -213,7 +213,8 @@ fi
 set -- ${DOCKER_ARGS}
 
 for var in SHELL TERM KAS_DISTRO KAS_MACHINE KAS_TARGET KAS_TASK \
-           http_proxy https_proxy ftp_proxy no_proxy NO_PROXY; do
+           http_proxy https_proxy ftp_proxy no_proxy NO_PROXY \
+           KAS_PREMIRRORS; do
 	if [ -n "$(eval echo \$${var})" ]; then
 		set -- "$@" -e "${var}='$(eval echo \"\$${var}\")'"
 	fi