uboot-sign: fix the concatenation when multiple U-BOOT configurations are specified

Some BSPs, especially those of meta-freescale and meta-ti allow to build U-Boot
binaries using different configuration for a given target, for example:
- UBOOT_CONFIG ??= "tfa-secure-boot tfa"
- UBOOT_CONFIG ??= "nand sdcard spi nor"

When this is the case the public key wasn't concatenated to all U-Boot binaries
built.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>