hush: add CONFIG_HUSH_PARSER to Kconfig

hush: add CONFIG_HUSH_PARSER to Kconfig

The README file states that the macros beginning with "CONFIG_SYS_"
depend on the hardware etc. and should not be meddled with if you do
not what you're doing.
We have already screwed up with this policy; we have given the prefix
"CONFIG_SYS_" to many user-selectable configurations.
Here, "CONFIG_SYS_HUSH_PARSER" is one of them.  Users can enable it
if they want to use a more powerful command line parser, or disable it
if they only need a simple one.

This commit attempts to rename CONFIG_SYS_HUSH_PARSER to
CONFIG_HUSH_PARSER and move it to Kconfig.

Every board maintainer is expected to enable CONFIG_HUSH_PARSER
(= add "CONFIG_HUSH_PARSER=y" to his defconfig file) and remove
"#define CONFIG_SYS_HUSH_PARSER" from his header file.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>