spl: Allow performing BSS init early before board_init_f()

On some platform we have sufficient memory available early on to allow
setting up and using a basic BSS prior to entering board_init_f(). Doing
so can for example be used to carry state over to board_init_r() without
having to resort to extending U-Boot's global data structure.

To support such scenarios add a Kconfig option called CONFIG_SPL_EARLY_BSS
to allow moving the initialization of BSS prior to entering board_init_f(),
if enabled. Note that using this option usually should go along with using
CONFIG_SPL_SEPARATE_BSS and configuring BSS to be located in memory
actually available prior to board_init_f().

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>