Allow the default #defines for sdmalloc/sdfree to be overridden by
system-specific versions.  This can be used to avoid extra copying
by making sure sd buffers are cache-aligned (some ARM systems) or
page-aligned (xen) for DMA.

Reference: /n/sources/patch/applied/sdmalloc-override
Date: Tue Oct  2 21:10:45 CES 2012
Signed-off-by: miller@hamnavoe.com

--- /sys/src/9/port/sd.h	Tue Oct  2 21:05:04 2012
+++ /sys/src/9/port/sd.h	Tue Oct  2 21:05:02 2012
@@ -134,8 +134,10 @@
 	SDnpart		= 16,
 };
 
+#ifndef sdmalloc
 #define sdmalloc(n)	malloc(n)
 #define sdfree(p)	free(p)
+#endif
 
 /* devsd.c */
 extern void sdadddevs(SDev*);