spl: Make spl_boot_list a local variable

There is no need for this to be in the BSS region. By moving it we can delay
use of BSS in SPL. This is useful for machines where the BSS region is not
in writeable space. On 64-bit x86, SPL runs from SPI flash and it is easier
to eliminate BSS use than link SPL to run with BSS at a particular
cache-as-RAM (CAR) address.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>