arm: mxs: Preprocess u-boot.bd so they contain full path

The u-boot-imx23.bd and u-boot-imx28.bd need to be preprocessed, otherwise
they have issues with out-of-tree build where elftosb tool couldn't sometimes
find the u-boot.bin and spl/u-boot-spl.bin .

Preprocess these .bd files with sed and insert full path to u-boot.bin and
spl/u-boot-spl.bin to prevent this issue. Moreover, to avoid adding more
churn into main Makefile, move all this preprocessing and u-boot.sb generation
into CPU directory instead.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>