otter: allow building ShiftOS

Ignore the shiftos/ directory, which is used for ShiftOS specific
builds and include shiftos/BoardConfig.mk if it exists.

Change-Id: I5f4f748c223bc1a24f57a8690dd2919c96f3e41d
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 4aba399..e57e05c 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -191,3 +191,6 @@
 
 # Inherit the proprietary files
 include vendor/shift/otter/BoardConfigVendor.mk
+
+# Include ShiftOS specific BoardConfig if existing
+-include device/shift/otter/shiftos/BoardConfig.mk