gen-grub-cfg: Support different kernel file names

Different architectures and/or configurations can use a different name
for the Linux kernel image file.  The `gen-grub-cfg.sh` shell script
needs to be able to identify the correct file name in the GRUB
configuration fragment it embeds in the root filesystem image.

boot/grub2/gen-grub-cfg.sh

@@ -3,10 +3,11 @@
 
 vers=${BR2_EXTERNAL_AIMEEOS_VERSION}
 br2_vers=${BR2_VERSION_FULL}
+kernel_filename=$1
 
 cat > "${TARGET_DIR}"/boot/grub.cfg <<EOF
 menuentry "Aimee OS ${vers} (Buildroot ${br2_vers}) on \$dev" --class gnu-linux --id "id-\$partuuid" "\$dev" "\$partuuid" {
     set root="\$2"
-    linux "/boot/zImage" root=PARTUUID=\$3 ro
+    linux "/boot/${kernel_filename}" root=PARTUUID=\$3 ro
 }
 EOF