uboot-sign.bbclass: fix some install commands

A "install -d" command is missing when dealing with
${UBOOT_NODTB_BINARY} in install_spl_helper function, this can lead
to invalid install error saying:

| install: cannot create .../usr/share/u-boot-nodtb-2021.04-r0.bin': No such file or directory

Let's drop all "install -d" and replace them with "install -Dm" in
install_helper/install_spl_helper functions.

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>