build: Revive and update LDR format support

LDR format files are used primarily by Analog Devices processors but may
be of interest to other vendors. Previously support existed for this
format as part of the U-Boot build, but it has been unmaintained and
unused for a long time. In preparation for adding support for modern ADI
processors that use LDR, modernize the LDR support:

- Introduce CONFIG_LDR_CPU as the CPU string recognized by the LDR tool
  may not be the same as CONFIG_CPU
- Add an SPL target that repackages u-boot-spl inside an LDR file

An almost identical target for packaging u-boot into an LDR file already
exists and did not need to be created.

Co-developed-by: Nathan Barrett-Morrison <nathan.morrison@timesys.com>
Signed-off-by: Nathan Barrett-Morrison <nathan.morrison@timesys.com>
Signed-off-by: Ian Roberts <ian.roberts@timesys.com>
Signed-off-by: Greg Malysa <greg.malysa@timesys.com>