terminal.bbclass: force bash for devshell

Since shell_trap_code in build.py sets /bin/sh as the interpreter
we will die a silent death if our environment contains things like
"export -f bodilyfunction" and /bin/sh is really /bin/dash.

Fixes this for the case of devshell.

Signed-off-by: Joe Slater <joe.slater@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>