efi: Avoid using non-existent text base

When U-Boot runs as an EFI application is does not have a definition of
CONFIG_SYS_TEXT_BASE. U-Boot is a relocatable application and the relocation
is done by EFI. U-Boot can be loaded at any address.

This is similar to how sandbox works. Adjust the early board init to deal
with this.

Signed-off-by: Ben Stoltz <stoltz@google.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>