arm: imx: Rework i.MX specific commands to be excluded from SPL

arm: imx: Rework i.MX specific commands to be excluded from SPL

The "clocks" and "bootaux" commands are only usable in full U-Boot, not
SPL, so do not link them inside of SPL.  Rework a little of the bootaux
related code to make use of __weak and declare parts of it static as
it's local to the file.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>