flashy-0ec2208

nexthopbmc: flashy: register nexthopbmc platform (#285)

Summary:
# Description

Add nexthopbmc to the flashy BMC-lite platform list, the flash procedure mappings (FlashCp) and platform_build_names so flashy recognizes and flashes nexthopbmc images.
# Motivation

nexthopbmc targets should be able to be flashed with the flashy tool.

Pull Request resolved: https://github.com/facebook/openbmc/pull/285

Test Plan:
Do an image upgrade:
```
./scripts/run_flashy_remote.sh \
      --host nexthopbmc-oob \
      --device mtd:flash0 \
      --imagepath /srv/tftp/sync/flash-nexthopbmc
Running a remote upgrade on 'nexthopbmc-oob'  with image '/srv/tftp/sync/flash-nexthopbmc'
Continue (y/n)?y
path-to-flashy not provided, building flashy...
Making installation directories on OpenBMC...
Copying flashy...
Copying image...
Copying upgrade script...
Running upgrade...
Getting buildname of device
Buildname: nexthopbmc
<snip>
2026/09/08 07:07:10 Fetching image data for offset: 33554432, size: 13238
2026/09/08 07:07:10 Flashing image data for offset: 33554432, size: 13238
2026/09/08 07:07:11 Finished flashing image '/run/upgrade/image' on to flash device '/dev/mtd5'
2026/09/08 07:07:11 Verifying copy on flash device '/dev/mtd5' with image file '/run/upgrade/image'
2026/09/08 07:07:16 Finished verifying copy on flash device '/dev/mtd5' with image file '/run/upgrade/image'
2026/09/08 07:07:16 FlashCp succeeded
2026/09/08 07:07:16 Finished: flashy
Flashing succeeded. It is safe to reboot this system. Flashing complete, rebooting the device...
Connection to nexthopbmc-oob closed by remote host.
```

Flashy go tests pass:
```
nate@builder:~/bmc/nh-openbmc/tools/flashy$ go test 2026/09/08 14:35:18 Running command
'/home/nate/bmc/nh-openbmc/tools/flashy/scripts/calculate_test_coverage.sh' with 30s timeout
2026/09/08 14:35:19 stdout: 93.9
2026/09/08 14:35:19 Command
'/home/nate/bmc/nh-openbmc/tools/flashy/scripts/calculate_test_coverage.sh' exited with code 0 after 915.914042ms
PASS
ok      github.com/facebook/openbmc/tools/flashy        0.918s
```

Reviewed By: joancaneus

Differential Revision: D119173908

Pulled By: ezeob002

fbshipit-source-id: 8b60924bdfb1d67800a49455fc2782cf06111c6e