linux: replace shell for loop with make foreach loop

This commit replaces the loop copying out-of-tree DTS into the kernel
tree by a make foreach loop instead of a shell for loop. This allows
to error out if one of the DTS file cannot be copied (for example if
it doesn't exist).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>