luci-app-advanced-reboot: bugfix: actual partition switch

luci-app-advanced-reboot: bugfix: actual partition switch

At some point after r7, when I no longer had the dual boot device readily
available for testing, a regression was introduced that the RPCD script
would not actually perform the partition switch.
Thanks to @bd0426 for report and @Jackie264 for fixing the issue.

advanced-reboot.js
* simplify partition number validation in handleAlternativeReboot
* reformat text copy

RPCD script
* obtain OpenWrt version from "$OPENWRT_RELEASE" not "$PRETTY_NAME" for
  the snapshot compatibility
* refactor parameter processing for boot_partition function
* add logging/better output on error in boot_partition

test.sh
* introduce a test file for heartbeat check of the RPCD script

Signed-off-by: Stan Grishin <redacted>