lbmk: simplify execute_command()

if ./build command options fails, it just means that
lbmk would next check whether ./buildpath mode list exists,
which it never will because that would violate lbmk design.

the generic "help" output is more than sufficient, and tells
the user to check "list" anyway, so there's no point in saying
it here. simplify this function.

Signed-off-by: Leah Rowe <leah@libreboot.org>

lbmk

@@ -93,20 +93,14 @@ install_packages()
 
 execute_command()
 {
-	case "${option}" in
-	list)
-		./build command options "${buildpath}/${mode}" ;;
-	*)
-		if [ ! -d "${buildpath}/${mode}" ]; then
-			usage $0
-			fail "Invalid mode '${mode}'. Run: ${0} help"
-		elif [ ! -f "${buildpath}/${mode}/${option}" ]; then
-			usage $0
-			printf "Invalid option for '%s'." ${mode} 1>&2
-			fail "Run: ${0} ${mode} list'."
-		fi
+	if [ "${option}" = "list" ]; then
+		./build command options "${buildpath}/${mode}"
+		lbmk_exit 0
+	elif [ ! -f "${buildpath}/${mode}/${option}" ]; then
+		fail "Invalid command. Run: ${linkpath} help"
+	else
 		"${buildpath}/${mode}/${option}" $@ || fail "lbmk error"
-	esac
+	fi
 }
 
 usage()