spl: add hierarchical defaults for SPL_LDSCRIPT

With SPL_LDSCRIPT moved to Kconfig (and this being a 'string' config
node), all the lingering definitions in header files will cause
warnings/errors due to the redefinition of the configuration item.

As we don't want to pollute the defconfig files (and values should
usually be identical for entire architectures), the defaults are moved
into Kconfig.  Kconfig will always pick the first default that
matches, so please keep these values at the end of each file (to allow
any board-specific Kconfig, which will be included earlier) to
override with an unconditional default setting.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>