Kconfig.boot: Make 0x0 the default SYS_TEXT_BASE for POSITION_INDEPENDENT

When we build U-Boot with POSITION_INDEPENDENT we must have
SYS_TEXT_BASE be set to zero.  Make this the default in that case.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>