bash: fix static build

The static build needs some trickery, so make it happen. Fixes:
http://autobuild.buildroot.net/results/2b12868f8297a96d7abf7fcf526ca5eace73d66d/



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