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>

include/image.h

@@ -451,6 +451,7 @@ int image_check_dcrc (const image_header_t *hdr);
 int getenv_yesno (char *var);
 ulong getenv_bootm_low(void);
 phys_size_t getenv_bootm_size(void);
+phys_size_t getenv_bootm_mapsize(void);
 void memmove_wd (void *to, void *from, size_t len, ulong chunksz);
 #endif