[pc] Use cbuf lib instead of custom queue.

The facilities to manage a key buffer with non-busy waiting already
exist, so use the existing lib in the keyboard driver instead of the
custom circular queue and wait queue.
diff --git a/platform/pc/rules.mk b/platform/pc/rules.mk
index e083a80..e10fb1b 100644
--- a/platform/pc/rules.mk
+++ b/platform/pc/rules.mk
@@ -3,6 +3,9 @@
 ARCH := x86
 CPU := generic
 
+MODULES += \
+	lib/cbuf
+
 INCLUDES += \
 	-I$(LOCAL_DIR)/include