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

@@ -29,7 +29,6 @@ AOBJS	=
 
 # core
 COBJS-y += main.o
-COBJS-y += circbuf.o
 COBJS-y += console.o
 COBJS-y += command.o
 COBJS-y += dlmalloc.o

lib_generic/Makefile

@@ -31,6 +31,7 @@ COBJS-$(CONFIG_BZIP2) += bzlib_crctable.o
 COBJS-$(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