externalgitsrc: changed so that the variable is now only set once instead of beeing read every time its assigned.

This also rebuilds recipes which use the EXTERNALSRC_GIT_SRCREV variable, when the variable changes.

Signed-off-by: Lucien Mueller <lucien.mueller@netmodule.com>

classes/externalgitsrc.bbclass

@@ -1,7 +1,6 @@
 # Provides git describe tag from yocto root directory
 
 EXTERNALSRC = "${@os.path.abspath(os.path.join("${TOPDIR}", os.pardir))}"
-EXTERNALSRC_GIT_SRCREV = "${@get_externalsrc_rev(d)}"
 
 def get_externalsrc_rev(d):
     import subprocess
@@ -14,3 +13,4 @@ def get_externalsrc_rev(d):
     d.setVar('BB_DONT_CACHE', '1')
     return rev
 
+EXTERNALSRC_GIT_SRCREV := "${@get_externalsrc_rev(d)}"