cli: Reduce size of readline buffers in SPL

Normally, readline is not used int SPL. However, it may be useful to
enable the Freescale DDR interactive mode in SPL, while U-Boot is still
executing from SRAM. The default settings for readline result in a large
buffer being allocated. Reduce the size of the maximum input line, and
the number of lines of scrollback when building for SPL.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>