hush: add showvar command for hush shell.

This new command shows the local variables defined in
the hush shell:

=> help showvar
showvar
    - print values of all hushshell variables
showvar name ...
    - print value of hushshell variable 'name'

Also make the set_local_var() and unset_local_var ()
no longer static, so it is possible to define local
hush shell variables at boot time. If CONFIG_HUSH_INIT_VAR
is defined, u-boot calls hush_init_var (), where
boardspecific code can define local hush shell
variables at boottime.

Signed-off-by: Heiko Schocher <hs@denx.de>