Redesign EFI boot

- concatenating two gzipped cpio archives doesn't work, so we need to do it
  differently

embed.ipxe

@@ -142,16 +142,14 @@ iseq ${platform} efi && goto is_efi || goto not_efi
 # Attributes for EFI boot
 :is_efi
 # hack, see https://github.com/coreos/fedora-coreos-tracker/issues/390#issuecomment-588328137
-set squashfs squashfs.cpio.gz
-set initrd1 initrd=boot-initrd
-set initrd2 initrd=squashfs.cpio.gz
+clear squashfs
+set initrd initrd_squashfs.cpio.gz
 goto get_arch
 
 # this sets different attributes for non-EFI (legacy PC) boot
 :not_efi
 set squashfs squashfs
-set initrd1 initrd=boot-initrd
-clear initrd2
+set initrd boot-initrd
 goto get_arch
 
 
@@ -210,6 +208,8 @@ goto static_ip_boot_setup
 # However, note that imgverify will fail if you refer to it as "/squashfs"
 #   instead of "squashfs".
 
+isset ${squashfs} || goto skip_squashfs
+
 imgfetch http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/${squashfs} /${squashfs} || goto try_next_card
 imgverify --signer images.sysdeploy.org ${squashfs} http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/${squashfs}.sig || goto error_handler
 
@@ -221,8 +221,10 @@ imgverify --signer images.sysdeploy.org ${squashfs} http://images.sysdeploy.org/
 #   make sure you pick a common name with a FQDN you control, even if you're
 #   using a custom CA that you import during build.
 
-initrd http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/boot-initrd || goto try_next_card
-imgverify --signer images.sysdeploy.org boot-initrd http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/boot-initrd.sig || goto error_handler
+:skip_squashfs
+
+initrd http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/${initrd} || goto try_next_card
+imgverify --signer images.sysdeploy.org boot-initrd http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/${initrd}.sig || goto error_handler
 
 kernel http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/boot-kernel || goto try_next_card
 imgverify --signer images.sysdeploy.org boot-kernel http://images.sysdeploy.org/${ubuntu-variant}/${arch-info}/boot-kernel.sig || goto error_handler
@@ -235,7 +237,7 @@ echo ip-info ${ip-info}
 
 sleep 1
 
-boot boot-kernel ${initrd1} ${initrd2} rootfstype=squashfs root=/squashfs ip=${ip-info} overlayroot=tmpfs:recurse=0 systemd.clock-usec=${unixtime:int32}000000 ds=nocloud-net;s=https://cloud-init.sysdeploy.org/ || goto error_handler
+boot boot-kernel initrd=${initrd} rootfstype=squashfs root=/squashfs ip=${ip-info} overlayroot=tmpfs:recurse=0 systemd.clock-usec=${unixtime:int32}000000 ds=nocloud-net;s=https://cloud-init.sysdeploy.org/ || goto error_handler
 
 # unixtime variable must be used with int32, because that's the only way it
 #   will display as decimal digits. unit32 and string both display as hex.