circbuf: Move to lib_generic and conditionally compile

circbuf could be used as a generic library and is only currently
needed when CONFIG_USB_TTY is defined.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

common/Makefile

diff --git a/common/Makefile b/common/Makefile
index 3781738..47f6a71 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -29,7 +29,6 @@
 
 # core
 COBJS-y += main.o
-COBJS-y += circbuf.o
 COBJS-y += console.o
 COBJS-y += command.o
 COBJS-y += dlmalloc.o

lib_generic/Makefile

diff --git a/lib_generic/Makefile b/lib_generic/Makefile
index 686601c..2ec261a 100644
--- a/lib_generic/Makefile
+++ b/lib_generic/Makefile
@@ -31,6 +31,7 @@
 COBJS-$(CONFIG_BZIP2) += bzlib_decompress.o
 COBJS-$(CONFIG_BZIP2) += bzlib_randtable.o
 COBJS-$(CONFIG_BZIP2) += bzlib_huffman.o
+COBJS-$(CONFIG_USB_TTY) += circbuf.o
 COBJS-y += crc16.o
 COBJS-y += crc32.o
 COBJS-y += ctype.o

lib_generic/circbuf.c

diff --git a/common/circbuf.c b/lib_generic/circbuf.c
similarity index 100%
rename from common/circbuf.c
rename to lib_generic/circbuf.c