sh: Convert the last use of 'optional' property in Kconfig

The 'choice' statement is primarily used to exclusively select one
option, but the 'optional' property allows all entries to be disabled.

This feature is rarely used. In fact, it is only used in arch/sh/Kconfig
because the equivalent outcome can be achieved by inserting one more
entry.

The 'optional' property support will be removed from Kconfig.

This commit replaces the 'optional' property with a dummy option,
CMDLINE_FROM_BOOTLOADER, as seen in some other architectures.

Note:
 The 'default CMDLINE_OVERWRITE' statement does not work as intended
 in combination with 'optional'. If neither CONFIG_CMDLINE_OVERWRITE
 nor CONFIG_CMDLINE_EXTEND is specified in a defconfig file, both of
 them are disabled. This is a bug. To maintain the current behavior,
 I added CONFIG_CMDLINE_FROM_BOOTLOADER=y to those defconfig files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>