uboot-sign: Fix build when UBOOT_DTB_BINARY is empty
When UBOOT_DTB_BINARY is empty and because the code now changes
directory into ${B}, the test for the existence becomes `[ -f ]` which
succeeds and subsequently the install fails.
Reorder the code so it's clear that UBOOT_DTB_BINARY empty is an
expected configuration and then quote UBOOT_DTB_BINARY everywhere so
no one trips over this again.
Fixes: bacb59079eb6 ("uboot-sign: add support for different u-boot configurations") Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
