Default to bootm_size() when CONFIG_SYS_BOOTMAPSZ is not defined

This patch adds a function getenv_bootm_mapsize() for obtaining the
size of the early mapped region accessible by the kernel during early
boot.  It defaults to CONFIG_SYS_BOOTMAPSZ, or if not defined,
defaults to getenv_bootm_size(), which in turn defaults to the size of
RAM.

getenv_bootm_mapsize() can also be overridden with a "bootm_mapsize"
environmental variable.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
diff --git a/arch/powerpc/lib/bootm.c b/arch/powerpc/lib/bootm.c
index 6e2a81b..02e544f 100644
--- a/arch/powerpc/lib/bootm.c
+++ b/arch/powerpc/lib/bootm.c
@@ -96,7 +96,7 @@
 		debug ("   Booting using OF flat tree...\n");
 		WATCHDOG_RESET ();
 		(*kernel) ((bd_t *)of_flat_tree, 0, 0, EPAPR_MAGIC,
-			   CONFIG_SYS_BOOTMAPSZ, 0, 0);
+			   getenv_bootm_mapsize(), 0, 0);
 		/* does not return */
 	} else
 #endif