add MKPREFIX flag to build system

This allows building OpenRC for a Gentoo Prefix installation.

README

@@ -12,6 +12,7 @@ LIBNAME=lib64
 DESTDIR=/tmp/openrc-image
 MKNET=oldnet
 MKPAM=pam
+MKPREFIX=yes
 MKPKGCONFIG=no
 MKSELINUX=yes
 MKSTATICLIBS=no
@@ -25,6 +26,8 @@ We don't support building a static OpenRC with PAM.
 You may need to use PROGLDFLAGS=-Wl,-Bstatic on glibc instead of just -static.
 If you debug memory under valgrind, add -DDEBUG_MEMORY to your CPPFLAGS
 so that all malloc memory should be freed at exit.
+If you are building OpenRC for a Gentoo Prefix installation, add
+MKPREFIX=yes.
 
 You can also brand OpenRC if you so wish like so
 BRANDING=\"Gentoo/$(uname -s)\"