cmd: increase default for SYS_MAXARGS

The value of CONFIG SYS_MAXARGS limits the usability of the 'for' command.
The current default of 16 is too low for some use case. Cf.
https://bugs.launchpad.net/snap-core18/+bug/1910094



Increase the default to 64.

Reported-by: Dave Jones <dave.jones@canonical.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>