arm: mach-imx: Makefile: Extend u-boot-nand.imx padding

arm: mach-imx: Makefile: Extend u-boot-nand.imx padding

Extend the padding process of u-boot-nand.imx target by adding 10k bytes
of zeros to the end of the binary using the 'dd' command.

The existing padding method did not generate a functional binary,
as discussed in more detail in this thread [1]. Instead, we adopt the
end-padding calculation method documented in 'board/doc/colibri_imx7.rst'
as a reference, which is relevant for iMX7 with NAND storage.

Adding 10k bytes of zeros provides an approximate value that makes the
proper padding for these NAND devices.

[1] https://lore.kernel.org/all/CAC4tdFUqffQzRQFv5AGe_xtbFy1agr2SEpn_FzEdexhwjdryyw@mail.gmail.com/

Signed-off-by: Hiago De Franco <hiago.franco@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>