image: check "bootm_low" and "bootm_size" if "initrd_high" is missing

image: check "bootm_low" and "bootm_size" if "initrd_high" is missing

To boot Linux, we should prevent Initramdisk and FDT from going too
high.

Currently, boot_relocate_fdt() checks "fdt_high" environment first,
and then falls back to getenv_bootm_mapsize() + getenv_bootm_low()
if "fdt_high" is missing.

On the other hand, boot_ramdisk_high() only checks "initrd_high" to
get the address limit for the Initramdisk.  We also want to let this
case fall back to getenv_bootm_mapsize() + getenv_bootm_low().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>