Change to use kernel not bzImage everywhere

Previously we hardcoded `bzImage` which is not used for all
use cases or architectures.

fix #1630

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

tools/mkimage-gcp/make-gcp

@@ -5,7 +5,7 @@ set -e
 mkdir -p /tmp/image
 cd /tmp/image
 
-# input is a tarball of vmlinuz64 and initrd.img on stdin
+# input is a tarball of kernel and initrd.img on stdin
 # output is a compressed tarball of a raw disk image on stdout
 
 mkdir -p files
@@ -17,10 +17,10 @@ cd files
 [ -t 0 ] || tar xf -
 
 INITRD="$(find . -name '*.img')"
-KERNEL="$(find . -name vmlinuz64 -or -name bzImage)"
+KERNEL="$(find . -name kernel -or -name bzImage)"
 CMDLINE="$*"
 
-[ "$KERNEL" = "./vmlinuz64" ] || mv "$KERNEL" vmlinuz64
+[ "$KERNEL" = "./kernel" ] || mv "$KERNEL" kernel
 [ "$INITRD" = "./initrd.img" ] || mv "$INITRD" initrd.img
 
 # clean up subdirectories
@@ -31,7 +31,7 @@ GCP_CONFIG="earlyprintk=ttyS0,115200 console=ttyS0,115200 vsyscall=emulate page_
 
 CFG="DEFAULT linux
 LABEL linux
-    KERNEL /vmlinuz64
+    KERNEL /kernel
     INITRD /initrd.img
     APPEND ${CMDLINE}
 "