lib: add crc7 from Linux

Crc7 is used to compute mmc spi command packet checksum.

Copy from linux-2.6 lib/crc7.c include/linux/crc7.h
commit ad241528c4919505afccb022acbab3eeb0db4d80

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
diff --git a/lib/Makefile b/lib/Makefile
index ffdee7d..fcfe351 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -32,6 +32,7 @@
 COBJS-$(CONFIG_BZIP2) += bzlib_randtable.o
 COBJS-$(CONFIG_BZIP2) += bzlib_huffman.o
 COBJS-$(CONFIG_USB_TTY) += circbuf.o
+COBJS-y += crc7.o
 COBJS-y += crc16.o
 COBJS-y += crc32.o
 COBJS-y += ctype.o