Fix 'make clean' and .gitignore for firmware/ directory.

Provide a .gitignore file, and fix a typo which prevented some of the
generated binary files from being removed on 'make clean'.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Reported-and-tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/firmware/.gitignore b/firmware/.gitignore
new file mode 100644
index 0000000..d9c6901
--- /dev/null
+++ b/firmware/.gitignore
@@ -0,0 +1,6 @@
+*.gen.S
+*.fw
+*.bin
+*.csp
+*.dsp
+ihex2fw