axolotl: allow building ShiftOS

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

Change-Id: I397692d3a4982118c732cdec325690cdc5eb54f8
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..4cdddd9
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/shiftos/