scripts: use env to invoke bash

Not all systems have bash at a fixed location like /bin/bash.
FreeBSD, for example, would typically have it at /usr/local/bin/bash.
When building OpenSBI on freebsd system, the build breaks.

Its advisable to use: #!/usr/bin/env bash

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>