package/bash: add /bin/bash to /etc/shells

package/bash: add /bin/bash to /etc/shells

When bash is selected, /bin/bash is not added to /etc/shells
(see man shells). So, login tools like dropbear reject the ssh
connexions for users using bash as shell in /etc/passwd.

buildroot authpriv.warn dropbear[853]: User 'kubu' has invalid shell, rejected

Reported-by: Jeremy Rosen <jeremy.rosen@smile.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>