nixos/xen: dehardcode the .pad section from the UKI builder

Upstream, intentionally or not, no longer appends the EFI image
with a .pad section for us to hook the rest of the UKI to. This
simply dehardcodes .pad from the awk script, instead using the
very last section in the binary. (Currently .reloc)

Co-authored-by: Yaroslav Bolyukin <iam@lach.pw>
Signed-off-by: Fernando Rodrigues <alpha@sigmasquadron.net>

nixos/modules/virtualisation/xen-boot-builder.sh

@@ -1,6 +1,8 @@
 # This script is called by ./xen-dom0.nix to create the Xen boot entries.
 # shellcheck shell=bash
 
+export LC_ALL=C
+
 # Handle input argument and exit if the flag is invalid. See virtualisation.xen.efi.bootBuilderVerbosity below.
 [[ $# -ne 1 ]] && echo -e "\e[1;31merror:\e[0m xenBootBuilder must be called with exactly one verbosity argument. See the \e[1;34mvirtualisation.xen.efi.bootBuilderVerbosity\e[0m option." && exit 1
 case "$1" in
@@ -89,7 +91,8 @@ EOF
         # https://xenbits.xenproject.org/docs/unstable/misc/efi.html.
         [ "$1" = "debug" ] && echo -e "\e[1;34mxenBootBuilder:\e[0m making Xen UKI..."
         xenEfi=$(jq -re '."org.xenproject.bootspec.v1".xen' "$bootspecFile")
-        padding=$(objdump --header --section=".pad" "$xenEfi" | awk '/\.pad/ { printf("0x%016x\n", strtonum("0x"$3) + strtonum("0x"$4))};')
+        finalSection=$(objdump --header --wide "$xenEfi" | tail -n +6 | sort --key="4,4" | tail -n 1 | grep -Eo '\.[a-z]*')
+        padding=$(objdump --header --section="$finalSection" "$xenEfi" | awk -v section="$finalSection" '$0 ~ section { printf("0x%016x\n", and(strtonum("0x"$3) + strtonum("0x"$4) + 0xfff, compl(0xfff)))};')
         [ "$1" = "debug" ] && echo "               - padding: $padding"
         objcopy \
             --add-section .config="$tmpCfg" \