terminal.bbclass: use var-SHELL as the shebang of wrapper script
The devshell.bbclass set var-SHELL to var-DEVSHELL, and terminal.bbclass
initial var-SHELL with `bash'. Keep sync with it, use var-SHELL rather
than hardcoded `/bin/sh' as the shebang of wrapper script.
On Ubuntu host, default shell is dash (/bin/sh -> dash), even though
we assign var-SHELL with `/bin/bash', the wrapper script is still dashism.
Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
