shell: Add option to keep current environment

When doing interactive development via kas shell it is often desirable
to keep the user's customized configuration. The new --preserve-env
argument has been added to support this scenario via an opt in flag.

This flag is blocked  when not running from a TTY or via kas-container
and kas issues a warning to the user about potential unintended side
effects when invoked.

Signed-off-by: Ryan Fairfax <rfairfax@linux.microsoft.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas-container

@@ -289,6 +289,10 @@ while [ $# -gt 0 ] && [ $KAS_EXTRA_BITBAKE_ARGS -eq 0 ]; do
 		KAS_BITBAKE_C_OPTION_ARGS="$2"
 		shift 2
 		;;
+	-E|--preserve-env)
+		echo "$1 is not supported with kas-container"
+		exit 1
+		;;
 	--)
 		KAS_EXTRA_BITBAKE_ARGS=$#
 		;;