AK3: fix unpack/repack with magiskboot zimage kernel support

- exclude "zimage" as a compression type since magiskboot unpack, split and repack handle it automatically and there is no fallback
- ensure $comp is a working command for all busybox compression fallbacks

META-INF/com/google/android/updater-script

@@ -3,4 +3,4 @@
 # Dummy file; update-binary is a shell script (DO NOT CHANGE)
 #
 #
-# AK_BASE_VERSION=20220310
+# AK_BASE_VERSION=20220403

tools/ak3-core.sh

@@ -320,9 +320,9 @@ flash_boot() {
         fi;
         if [ $((magisk_patched & 3)) -eq 1 ]; then
           ui_print " " "Magisk detected! Patching kernel so reflashing Magisk is not necessary...";
-          comp=$($bin/magiskboot decompress kernel 2>&1 | grep -v 'raw' | sed -n 's;.*\[\(.*\)\];\1;p');
+          comp=$($bin/magiskboot decompress kernel 2>&1 | grep -vE 'raw|zimage' | sed -n 's;.*\[\(.*\)\];\1;p');
           ($bin/magiskboot split $kernel || $bin/magiskboot decompress $kernel kernel) 2>/dev/null;
-          if [ $? != 0 -a "$comp" ]; then
+          if [ $? != 0 -a "$comp" ] && $comp --help 2>/dev/null; then
             echo "Attempting kernel unpack with busybox $comp..." >&2;
             $comp -dc $kernel > kernel;
           fi;
@@ -332,7 +332,7 @@ flash_boot() {
           fi;
           if [ "$comp" ]; then
             $bin/magiskboot compress=$comp kernel kernel.$comp;
-            if [ $? != 0 ]; then
+            if [ $? != 0 ] && $comp --help 2>/dev/null; then
               echo "Attempting kernel repack with busybox $comp..." >&2;
               $comp -9c kernel > kernel.$comp;
             fi;