image: fix getenv_bootm_size() function again

Commit 9c11135c ("image: fix getenv_bootm_size() function") fixed
the case where "bootm_low" is defined, but "bootm_size" is not.
Instead, it broke the case where neither "bootm_low" nor "bootm_size"
is defined.  Fix this function again.

Fixes: 9c11135c ("image: fix getenv_bootm_size() function")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Matthias Weisser <m.weisser.m@gmail.com>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>