uboot-imx

mx7_common: Define CONFIG_SYS_MALLOC_LEN in the board file

Having CONFIG_SYS_MALLOC_LEN in mx7_common.h is not a good idea,
because the malloc() pool size is board dependent.

For example: if a certain board has support for splashscreen or DFU,
it may be necessary to adjust CONFIG_SYS_MALLOC_LEN to a larger value.

So define CONFIG_SYS_MALLOC_LEN in each board config file.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>