Fix windows compilation

Path concatenation was wrong. Layout path was received as full path
and was concatenated to script directory path

Fix: do not prepend script directory path in case full path is received.

Issue reproduced on MSYS2

Change-Id: I5f627b5befcc56f4487dbfaaa1e2d9e1dd655103
Signed-off-by: Alexander Zilberkant <alexander.zilberkant@arm.com>
diff --git a/bl2/ext/mcuboot/scripts/assemble.py b/bl2/ext/mcuboot/scripts/assemble.py
index 9c48092..0bb41f1 100644
--- a/bl2/ext/mcuboot/scripts/assemble.py
+++ b/bl2/ext/mcuboot/scripts/assemble.py
@@ -44,8 +44,11 @@
         offsets = {}
         sizes = {}
 
-        scriptsDir = os.path.dirname(os.path.abspath(__file__))
-        configFile = os.path.join(scriptsDir, self.layout_path)
+        if os.path.isabs(self.layout_path):
+            configFile = self.layout_path
+        else:
+            scriptsDir = os.path.dirname(os.path.abspath(__file__))
+            configFile = os.path.join(scriptsDir, self.layout_path)
 
         with open(configFile, 'r') as fd:
             for line in fd: