[image] Check delimiters when parsing command-line key-value arguments

The Linux kernel bzImage image format and the CPIO archive constructor
will parse the image command line for certain arguments of the form
"key=value".  This parsing is currently implemented using strstr() in
a way that can cause a false positive suffix match.  For example, a
command line containing "highmem=<n>" would erroneously be treated as
containing a value for "mem=<n>".

Fix by centralising the logic used for parsing such arguments, and
including a check that the argument immediately follows a whitespace
delimiter (or is at the start of the string).

Reported-by: Filippo Giunchedi <filippo@esaurito.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/image.h

@@ -195,6 +195,7 @@ extern struct image * image_find_selected ( void );
 extern int image_set_trust ( int require_trusted, int permanent );
 extern struct image * image_memory ( const char *name, userptr_t data,
 				     size_t len );
+extern const char * image_argument ( struct image *image, const char *key );
 extern int image_pixbuf ( struct image *image, struct pixel_buffer **pixbuf );
 extern int image_asn1 ( struct image *image, size_t offset,
 			struct asn1_cursor **cursor );