busybox: enable fully featured hush shell for nommu

As pointed by Rich Fekler on IRC the basic hush shell is pretty much
useless since it doesn't support if conditionals, loops, functions, case
or even interactive mode.
So enable the full feature-set.
Size delta: +10184 bytes uncompressed for blackfin fdpic.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>