uboot-sign: fix U-Boot binary with public key

uboot-sign: fix U-Boot binary with public key

Fixes [YOCTO #15649]

The U-Boot binary in the "deploy" directory is missing the public key
when the removed logic branch is used.

The simple concatenation of the binary and DTB with public key works as
expected on a BeagleBone Black.

Given:
MACHINE = beaglebone-yocto
UBOOT_SIGN_KEYNAME = "dev"

Post-patch (poky/build/tmp/deploy/images/beaglebone-yocto):
$ hexdump -e "16 \"%_p\" \"\\n\"" u-boot-beaglebone-yocto.dtb \
| tr -d '\n' | grep -o 'key-dev'
key-dev

$ hexdump -e "16 \"%_p\" \"\\n\"" u-boot.img \
| tr -d '\n' | grep -o 'key-dev'
key-dev

Non-Poky BeagleBone Black testing (Scarthgap):
U-Boot 2024.01 [...]
[...]
Using 'conf-ti_omap_am335x-boneblack.dtb' configuration
Verifying Hash Integrity ... sha256,rsa4096:dev+ OK
Trying 'kernel-1' kernel subimage
[...]

Signed-off-by: Clayton Casciato <majortomtosourcecontrol@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>