hush: allow hush to run embedded scripts

hush: allow hush to run embedded scripts

Embedded scripts require a shell to be present in the BusyBox
binary.  Allow either ash or hush to be used for this purpose.
If both are enabled ash takes precedence.

The size of the binary is unchanged in the default configuration:
both ash and hush are present but support for embedded scripts
isn't compiled into hush.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>