kernel-devicetree: make shell scripts posix compliant

Use the notation suggested by Martin Jansa to avoid a bashism. Also
switch KERNEL_DTBVENDORED to the more common 0/1 notation instead of
true/false.

Fixes: 04ab57d2 (kernel-devicetree: allow specification of dtb
directory, 2023-05-02)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>