macports: Use needs_migration, chng migration msg

Re-use migration::needs_migration instead of re-implementing the check
in macports.tcl. Additionally, change the migration message to recommend
running 'sudo port migrate'.

src/macports1.0/macports.tcl

@@ -692,9 +692,9 @@ proc mportinit {{up_ui_options {}} {up_options {}} {up_variations {}}} {
         macosx_version
 
     # Check that the current platform is the one we were configured for, otherwise need to do migration
-    if {($os_platform ne $macports::autoconf::os_platform) || ($os_major != $macports::autoconf::os_major)} {
+    if {[migrate::needs_migration]} {
         ui_error "Current platform \"$os_platform $os_major\" does not match expected platform \"$macports::autoconf::os_platform $macports::autoconf::os_major\""
-        ui_error "If you upgraded your OS, please follow the migration instructions: https://trac.macports.org/wiki/Migration"
+        ui_error "Please run 'sudo port migrate' or follow the migration instructions: https://trac.macports.org/wiki/Migration"
         return -code error "OS platform mismatch"
     }