mx53loco: Add support to dynamically choose between fdt use or not

mx53loco: Add support to dynamically choose between fdt use or not

The CONFIG_SYS_CBSIZE has been change to 512 to avoid runtime errors as:

,---[ Runtime error ]
| Hit any key to stop autoboot:  0
| MX53LOCO U-Boot > pri netboot
| netboot=echo Booting from net ...; run netargs; if test ...
| prefetch abort
| pc : [<20747368>]          lr : [<20747365>]
| sp : af566e20  ip : 00000000     fp : 00000000
| r10: 00000002  r9 : af6dfc28     r8 : af566f58
| r7 : af6dfc10  r6 : 00000001     r5 : 00000002  r4 : 74206669
| r3 : 00000000  r2 : 00000060     r1 : 00000020  r0 : 0000018e
| Flags: nZCv  IRQs off  FIQs off  Mode SVC_32
| Resetting CPU ...
|
| resetting ...
`---

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>