linux: properly install all images in the initramfs case

linux: properly install all images in the initramfs case

The target "$(LINUX_DIR)/.stamp_initramfs_rebuilt" uses its own
'cp' command, instead of LINUX_INSTALL_IMAGE/LINUX_INSTALL_IMAGES_CMDS
provided by (or updated with) commit 055e6162bba7 ("linux: don't build
appended DTB image in place and support multiple images") and thus is
not operating properly when APPENDED_DTB is used.

Indeed, it copies a single image, and does not copy the one with the DTB
appended.

This patch replaces the 'cp' command with LINUX_INSTALL_IMAGE which
handles APPENDED_DTB.

Fixes: 055e6162bba7 ("linux: don't build appended DTB image in place and
support multiple images")

Signed-off-by: Sebastian Frias <sf84@laposte.net>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>