kdb: remove usage of static environment buffer

kdb: remove usage of static environment buffer

Problem: The set environment variable logic uses a static "heap" like
buffer to store the values of the variables, and they are never freed,
on top of that this is redundant since the kernel supplies allocation
facilities which are even used also in this file.

Solution: Remove the weird static buffer logic and use kmalloc instead,
call kfree when overriding an existing variable.

Signed-off-by: Nir Lichtman <nir@lichtman.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20250204054741.GB1219827@lichtman.org
Signed-off-by: Daniel Thompson <daniel@riscstar.com>