base-files: board_detect: make resilient against power-cuts

base-files: board_detect: make resilient against power-cuts

If board_detect is interrupted by cutting power on first boot,
board.json might only be half-way written and the file will not be
written again correctly on subsequent boots.

Write to a temporary file first, then rename. Since a rename on the same
file system is an atomic operation, it ensures that either
/etc/board.json does not exist or that the complete file exists.

Signed-off-by: Andreas Gnau <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20831
Signed-off-by: Hauke Mehrtens <[email protected]>