Fix: skip EEPROM config signing for naked provisioning (fixes #319)

[loonyfox1]

Problem

In 2.3.1, update_eeprom() signs the bootloader config (and, on 2712, bootcode/bootsys)
whenever a customer key is configured — regardless of PROVISIONING_STYLE. In naked mode
this produces an EEPROM containing a signed config block with an embedded public key,
while the device OTP remains unprogrammed (CUSTOMER_KEY_HASH all zeros). The resulting
device does not boot: solid red power LED, no green activity, no diagnostic blink code.
The provisioner reports EEPROM_UPDATE: success.

This is the root cause of #319 (reported on NVMe; reproduced identically on SD).

Evidence

Reproduced on two independent provisioning hosts (identical stacks: rpi-sb-provisioner
2.3.1, rpiboot 20260603~182203, rpi-eeprom 28.28-1), target: Raspberry Pi 5 Model B
8GB Rev 1.1 (BCM2712 C1, boardrev d04171), naked provisioning to SD.

Same firmware (pieeprom-2026-06-17.bin), same bootloader config, same board:

EEPROM write path	EEPROM_HASH	Boots
Manual rpiboot -d recovery5/ (unsigned)	4707d234…	✅
Provisioner 2.3.1 stock (signed config)	60bb7b48…	❌
Provisioner with this patch (unsigned)	4707d234…	✅

With the patch, the provisioner produces a byte-identical EEPROM to the known-good
manual usbboot recovery5 flash, and the device boots. The signing_available else-branch
already implements the correct unsigned flow; this change routes naked provisioning
through it, matching the existing case ${PROVISIONING_STYLE} pattern used elsewhere
in the script.

The log already hints at the inconsistency before writing the broken EEPROM:
Warning: SIGNED_BOOT=1 not found in ".../bootloader.naked" followed by
Signing bootloader config.

Notes

• Secure-boot and FDE flows are unaffected (condition only changes behaviour when
  PROVISIONING_STYLE=naked).
• Happy to provide full bootstrap logs and metadata JSONs from both the failing and
  passing runs if useful.