applypatch: Add a Makefile to build imgdiff in Chrome OS.

Also fixed some warnings and added check for target_len.

Test: mma; emerge-peppy imgdiff; emerge-nyan imgdiff; sudo emerge imgdiff
Bug: 26866274

Change-Id: Ifbcd3afd6701c769ccf626e33ed94461706f7ee6

applypatch/bsdiff.cpp

@@ -224,7 +224,6 @@ static void offtout(off_t x,u_char *buf)
 int bsdiff(u_char* old, off_t oldsize, off_t** IP, u_char* newdata, off_t newsize,
            const char* patch_filename)
 {
-	int fd;
 	off_t *I;
 	off_t scan,pos,len;
 	off_t lastscan,lastpos,lastoffset;