build.sh: add padding to flash.bin to achieve multiple of 512 bytes

Pad u-boot images with zeroes at the end to make the image size a multiple of 512 bytes. This allows faster reading of u-boot in 512 byte blocks when comparing the flashed contents and checksums with reform-flash-uboot. If the u-boot image has an arbitrary size, dd can only read it (and nothing beyond) with a blocksize of 1 because the blocksize also affects the seek and skip values.