boards/arm/cxd56xx/spresense: Add ostest, coremark and getprime configs.

[aviralgarg05]

Summary

Add dedicated Spresense defconfigs for ostest, coremark, and getprime.

This change is needed because the board already supports these workloads through broader configurations, but it did not have focused, standalone entry points for each one. The new configs keep the scope narrow and make it easier to run and validate the individual test or benchmark directly on Spresense.

What this change does:

• Adds boards/arm/cxd56xx/spresense/configs/ostest/defconfig
• Adds boards/arm/cxd56xx/spresense/configs/coremark/defconfig
• Adds boards/arm/cxd56xx/spresense/configs/getprime/defconfig
• Uses the existing Spresense board baseline and aligns the entry points to the expected app mains:
  • ostest_main
  • coremark_main
  • getprime_main

Fixes #17726

Impact

This is a board-configuration-only change.

User-facing impact:

• Spresense gains dedicated targets for OS testing and benchmarking.
• The new configs make it simpler to build, boot, and run the workloads independently.

Build/runtime impact:

• No source code or driver behavior changes.
• No documentation changes were required for this fix.
• No compatibility impact beyond adding new board configs.

Testing

Host:

• macOS, arm64

What I verified:

• Ran ./tools/configure.sh -L spresense and confirmed the new targets are listed:
  • spresense:ostest
  • spresense:coremark
  • spresense:getprime
• Ran the NuttX configure flow for each new config:
  • ./tools/configure.sh -E -a ../apps spresense:ostest
  • ./tools/configure.sh -E -a ../apps spresense:coremark
  • ./tools/configure.sh -E -a ../apps spresense:getprime
• Ran make olddefconfig and make savedefconfig for each config to confirm the checked-in defconfigs normalize correctly.
• Ran build attempts for all three configs.

Notes from the build verification:

• All three new configs reached the same toolchain failure seen on the existing Spresense baseline in this workspace.
• The failure happens in ./stdlib/lib_strtold.c with fatal error: math.h: No such file or directory.
• I also reproduced the same failure on the existing spresense:nsh baseline after the upstream sync, which indicates this is a workspace/toolchain issue rather than a regression from this change.

Relevant log summary:

• configure succeeded for all three configs
• olddefconfig succeeded for all three configs
• savedefconfig matched the intended board defconfigs, aside from the local host marker CONFIG_HOST_MACOS, which is not committed in NuttX board defconfigs
• make fails consistently at the same pre-existing point in this environment

[acassis]

Thank you @aviralgarg05 !!!