meta-evb: fix build warning for this layer

There are some build warning message as below:
WARNING: No bb files in default matched BBFILE_PATTERN_evb ''
WARNING: No bb files in default matched BBFILE_PATTERN_evb-nuvoton ''

Due to there is no any bb files in meta-evb and meta-evb-nuvoton layer.
Add this BBFILE_PATTERN_IGNORE_EMPTY to allow those layers has
no bb files then clear this kind of WARNING messages.

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: Ie845a2001b4e903975ab646ebe72aabd4f73e700
diff --git a/meta-evb/conf/layer.conf b/meta-evb/conf/layer.conf
index aa6b56f..445533f 100644
--- a/meta-evb/conf/layer.conf
+++ b/meta-evb/conf/layer.conf
@@ -3,4 +3,5 @@
 
 BBFILE_COLLECTIONS += "evb"
 BBFILE_PATTERN_evb = ""
+BBFILE_PATTERN_IGNORE_EMPTY_evb =  "1"
 LAYERSERIES_COMPAT_evb = "langdale mickledore"
diff --git a/meta-evb/meta-evb-nuvoton/conf/layer.conf b/meta-evb/meta-evb-nuvoton/conf/layer.conf
index f19d857..420306c 100644
--- a/meta-evb/meta-evb-nuvoton/conf/layer.conf
+++ b/meta-evb/meta-evb-nuvoton/conf/layer.conf
@@ -3,4 +3,5 @@
 
 BBFILE_COLLECTIONS += "evb-nuvoton"
 BBFILE_PATTERN_evb-nuvoton = ""
+BBFILE_PATTERN_IGNORE_EMPTY_evb-nuvoton =  "1"
 LAYERSERIES_COMPAT_evb-nuvoton = "langdale mickledore"