mkosi: Use distro-invariant rootprefix

Distributions may be build with various configs, e.g. customized
rootprefix. It'd be unmaintanable to have specific mkosi.build for each
supported distro. Hence, make the build script flexible yet distro
oblivious.

mkosi.build

@@ -42,6 +42,14 @@ fi
 if [ ! -f "$BUILDDIR"/build.ninja ] ; then
         sysvinit_path=`realpath /etc/init.d`
 
+        init_path=`realpath /sbin/init 2>/dev/null`
+        if [ -z "$init_path" ] ; then
+            rootprefix=""
+        else
+            rootprefix=${init_path%/lib/systemd/systemd}
+            rootprefix=/${rootprefix#/}
+        fi
+
         nobody_user=`id -u -n 65534 2> /dev/null`
         if [ "$nobody_user" != "" ] ; then
                 # Validate that we can translate forth and back
@@ -76,7 +84,7 @@ if [ ! -f "$BUILDDIR"/build.ninja ] ; then
                 fi
         fi
 
-        meson "$BUILDDIR" -D "sysvinit-path=$sysvinit_path" -D default-hierarchy=unified -D man=false -D "nobody-user=$nobody_user" -D "nobody-group=$nobody_group"
+        meson "$BUILDDIR" -D "sysvinit-path=$sysvinit_path" -D "rootprefix=$rootprefix" -D default-hierarchy=unified -D man=false -D "nobody-user=$nobody_user" -D "nobody-group=$nobody_group"
 fi
 
 ninja -C "$BUILDDIR" all