VPP requires bash for all shell scripts. Align shebang lines in build
and test scripts to look up the location of bash rather than hard coding
'/bin/bash'.
Look up the location of bash for makefiles.
Type: improvement
Change-Id: I23b705d81d60389fa8af61c680cf0abd74f0ea24
Signed-off-by: Tom Jones <thj@freebsd.org>