spl: Remove CONFIG_SPL_START_S_PATH and rework the logic behind it

spl: Remove CONFIG_SPL_START_S_PATH and rework the logic behind it

In some cases, when we don't use CONFIG_SPL_FRAMEWORK nor are we on
PowerPC using their specific SPL/TPL framework, we need to specify the
start.S file to use for these typically very constrained systems.  Do
this within the Makefile logic, rather than introducing a string-based
CONFIG option, as this would get slightly complex to do in Kconfig for a
very limited number of users.

Signed-off-by: Tom Rini <trini@konsulko.com>