Fix OneNAND ipl to read CONFIG_SYS_MONITOR_LEN

Add CONFIG_SYS_MONITOR_LEN macro to apollon board config.
CONFIG_SYS_MONITOR_LEN defines the U-Boot image size.
and is used by OneNAND ipl when reading U-Boot image.

Signed-off-by: Rohit Hagargundgi <h.rohit at samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>