env: Use static_assert() to check if default_environment is too large

env: Use static_assert() to check if default_environment is too large

Check sizeof(default_environment) against ENV_SIZE in a static_assert()
instead of runtime.
Only check if !USE_HOSTCC (for in fw_env tool ENV_SIZE expands to a
variable, and cannot be checked statically) nad
!DEFAULT_ENV_INSTANCE_EMBEDDED, for in that case the default_environment
variable is not set.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>