build: Restore install-user target

This helps users that may still expect it to exist. One example is
buildroot.

While at it, remove the now unused SUDO variable.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Makefile.am

 ACLOCAL_AMFLAGS=-I config
 
-SUDO=sudo
-
 SUBDIRS = 		\
 	doc		\
 	lib 		\
@@ -47,6 +45,9 @@ if XENO_COBALT
 	fi
 endif
 
+# legacy alias
+install-user: install
+
 install-exec-local: udev-rules
 
 .PHONY: FORCE