exynos: Increase command line buffer size (CONFIG_SYS_CBSIZE)

I was running into this limit with a not overly long PXE append line.

Since the PXE code wants to print the resulting command line increase
CONFIG_SYS_PBSIZE too.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>