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>