rockchip: back-to-bootrom: split BACK_TO_BOOTROM for TPL/SPL

The back-to-bootrom option is rather unfortunately named
  CONFIG_ROCKCHIP_SPL_BACK_TO_BOOTROM
instead of
  CONFIG_SPL_ROCKCHIP_BACK_TO_BOOTROM

To make is selectable through CONFIG_IS_ENABLED(ROCKCHIP_BACK_TO_BOOTROM),
we need to rename it.  At the same time, we introduce a TPL_ variant of
the option to give us finer-grained control over when it should be used.

This change is motivated by our RK3368 boot process, which returns to
the boot ROM only from the TPL stage, but not from the SPL stage.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
[added fix-up for evb-rk3229_defconfig and phycore-rk3288_defconfig:]
[fixed inverted CONFIG_IS_ENABLED test for rk3288:]
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/configs/rock.h: undef