[U-Boot] Default CONFIG_SYS_CBSIZE too low

[Tuomas Tynkkynen <tuomas@tuxera.com>]

Hi,

I was playing around with qemu_arm and qemu_arm64 and noticed some images
using the distro bootcmd infrastructure failed to boot there due to the buffer
for kernel command line arguments (which is determined by CONFIG_SYS_CBSIZE)
was too small. I found this odd given the images worked on some other boards
but turns out all of them bump it up from the default of 256:

include/configs/tegra-common.h:#define CONFIG_SYS_CBSIZE                (1024 * 2) /* Console I/O Buffer Size */
include/configs/sunxi-common.h:#define CONFIG_SYS_CBSIZE        1024    /* Console I/O Buffer Size */
include/configs/mx6_common.h:#define CONFIG_SYS_CBSIZE  512
include/configs/rpi.h:#define CONFIG_SYS_CBSIZE         1024

For 2018.03 I'll send a patch to increase it for qemu-arm too but I think
for the next release we should increase default (in config_fallbacks.h) so
that the user experience is consistent on all boards. But, I wonder how
safe it is to increase it for these boards low on RAM and stack? 
Should it perhaps be increased only for boards with CONFIG_DISTRO_DEFAULTS
that are expected to have enough resources for everything and the kitchen
sink? Or decoupling the kernel command line length from CONFIG_SYS_CBSIZE
and using malloc()? Other ideas?

Thanks,
- Tuomas