m68k: Fix warnings with gcc 4.6

Most of the warnings seem to be related to using 'int' for size_t. Change
this and fix up the remaining warnings and problems. For bootm, the warning
was masked by others, and there is an actual bug in the code.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/astro/mcf5373l/fpga.c b/board/astro/mcf5373l/fpga.c
index 1d044d9..d1110df 100644
--- a/board/astro/mcf5373l/fpga.c
+++ b/board/astro/mcf5373l/fpga.c
@@ -100,7 +100,7 @@
  * writing the complete buffer in one function is much faster,
  * then calling it for every bit
  */
-int altera_write_fn(void *buf, size_t len, int flush, int cookie)
+int altera_write_fn(const void *buf, size_t len, int flush, int cookie)
 {
 	size_t bytecount = 0;
 	gpio_t *gpiop = (gpio_t *)MMAP_GPIO;