kas-docker: made loop back device setup more idiot proof

I understood the message:

    Setting up loop device requires root privileges

as suggestion that I should start this script as root and hit
^C before `sudo` could prompt for my password.

This patch removes that line and adds a sudo prompt to elaborate on why
root permissions are needed and what is done with them. It also adds a
check if this was unsuccessful and suggests to the user to do it
themselves. This will also be shown if sudo is not installed.

Signed-off-by: Claudius Heine <ch@denx.de>

kas-docker

@@ -66,8 +66,17 @@ while [ $# -gt 0 ]; do
 				echo "Error: loop device not available!"
 				exit 1
 			fi
-			echo "Setting up loop device requires root privileges"
-			LOOP_DEV=$(sudo /sbin/losetup -f)
+			sudo_command="/sbin/losetup -f"
+			sudo_message="[sudo] enter password to setup loop"
+			sudo_message="$sudo_message devices by calling"
+			sudo_message="$sudo_message '$sudo_command': " 
+			if ! LOOP_DEV=$(sudo -p "$sudo_message" $sudo_command \
+				2>/dev/null); then
+				echo "Error: loop device setup unsuccessful!"
+				echo "try calling '$sudo_command' with root" \
+					"permissions manually."
+				exit 1
+			fi
 		fi
 		ISAR_ARGS="--cap-add=SYS_ADMIN --cap-add=MKNOD --privileged \
 			--device ${LOOP_DEV}"