grub-efi: Fix inability to upgrade to a different kernel.

Because we hardcode the name in the boot loader script, you can never
upgrade to a kernel with a different version string. Fix by using a
generic name and symlinking instead.

Changelog: Title

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>

mender-convert-modify

@@ -133,10 +133,13 @@ if [ "${MENDER_GRUB_EFI_INTEGRATION}" == "y" ]; then
 mender_rootfsa_part=${MENDER_ROOTFS_PART_A_NUMBER}
 mender_rootfsb_part=${MENDER_ROOTFS_PART_B_NUMBER}
 mender_grub_storage_device=${MENDER_GRUB_STORAGE_DEVICE}
-kernel_imagetype=${kernel_imagetype}
-initrd_imagetype=${initrd_imagetype}
+kernel_imagetype=kernel
+initrd_imagetype=initrd
 EOF
 
+  run_and_log_cmd "ln -s ${kernel_imagetype} work/rootfs/boot/kernel"
+  run_and_log_cmd "ln -s ${initrd_imagetype} work/rootfs/boot/initrd"
+
   # For partuuid support grub.cfg expects dedicated variables to be added
   if [ "${MENDER_ENABLE_PARTUUID}" == "y" ]; then
     rootfsa_partuuid=$(disk_get_partuuid_from_device "${root_part_a_device}")