x86: Ensure the e820 map is installed in all cases

This is a revert of a recent logic change in setup_zimage(). We do
actually need to install this information always. Change it to install
from the Coreboot tables if available, else the normal source.

Fixes: e7bae828 ("x86: Allow installing an e820 when booting from coreboot")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>