command: Allocate history buffer using calloc()

command: Allocate history buffer using calloc()

The history buffer is currently a static array which can be some
10-40 kiB depending on configuration, and so adds considerably to
the U-Boot binary size. Allocate it dynamically instead to reduce
the U-Boot binary size.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>