linux: only install the DTBs when not in appended DTB mode

When you're using the "appended DTB" mode, the Device Tree blob gets
appended to your kernel image, so there is no point in installing both
the DTB and the kernel image to the images or target directories,
installing the kernel image itself is sufficient.

Therefore, this commit disables the definition of LINUX_INSTALL_DTB
when appended DTB is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>