Added a small usability improve for Unix.mk savedefconfig

[ValentiWorkLearning]

Note: Please adhere to Contributing Guidelines.

Summary

I've found that while working on the custom board initial platform bringup it's a kind of:

• make menuconfig
• make savedefconfig
• git add
• git commit

When the board is configured to be out-of-tree, then it's possible to simply forget where the defconfig is stored/how to navigate into it.

Impact

Buildsystem

Testing

Just a simple make savedefconfig is enough:

"The defconfig was generated successfully at /workspaces/rtos_labs/nuttxspace/nuttx/defconfig .If you're building out-of-tree configuration, then copy it to your configuration defconfig"

[simbit18]

hi @ValentiWorkLearning replace Unix.ml with Unix.mk in the title

[ValentiWorkLearning]

Re-requested review just to ask, what should be done with this PR then 😄 Should I refactor it somehow or can we merge it?

[xiaoxiang781216]

Re-requested review just to ask, what should be done with this PR then 😄 Should I refactor it somehow or can we merge it?

@anchao do you have more concern?

[acassis]

@anchao could you please review/approve ?