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 <[email protected]>
Cc: Fabio Estevam <[email protected]>
Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Fabio Estevam <[email protected]>