buildroot

package/rpi-firmware: improve installation of dtb files

Don't list specific dtb files, instead install all dtb files in the same
way as dtbo files are currently done.

Signed-off-by: Martin Bark <martin@barkynet.com>
[Thomas: use make foreach loop instead of shell for loop.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>