linux: de-duplicate DTB and Linux image installation

Currently, the LINUX_INSTALL_DTB and LINUX_INSTALL_DTB_TARGET macros
are exactly the same, except for the target directory.

Similarly, LINUX_INSTALL_KERNEL_IMAGE_TO_TARGET and
LINUX_INSTALL_IMAGES_CMDS are copying the kernel image, just to a
different place (and with a different strategy).

As a preparation for future additions, this commit de-duplicate this
code:

 - LINUX_INSTALL_DTB becomes a make macro that takes one argument: the
   destination directory.

 - LINUX_INSTALL_IMAGE is a new make macro that also takes on
   argument: the destination directory.

Both macros are used by LINUX_INSTALL_KERNEL_IMAGE_TO_TARGET and
LINUX_INSTALL_IMAGES_CMDS to respectively install to the target
directory and the images directory.

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