fix(boot_patch): capture repack exit code and fix flash guard precedence (#153)

Found 4 distinct bugs in the boot_patch.sh that are inherited from
the APatch script this fork is based on:

1. Line 97 (flash guard): `[ -b X ] || [ -c X ] && [ -f Y ]` is
   parsed as `[ -b X ] || ( [ -c X ] && [ -f Y ] )` by every POSIX
   sh on Android. When BOOTIMAGE is a block device the
   `[ -f new-boot.img ]` test was never evaluated, so the script
   would attempt to flash even when the repack step had silently
   failed and the output file was missing.

2. Line 90 (repack error check): `if [ $? -ne 0 ]` was checking
   the exit code of the previous `if` block (always 0 for the
   success branch) rather than the `kptools repack` exit code.
   This meant repack failures were silently ignored.

3. Line 51: `exit $?` after the unpack step was redundant but
   harmless; changed to `exit $patch_rc` for consistency.

4. Line 78: same as #3 for the patch step.

Fix:
- Capture `./kptools repack` exit code into `repack_rc` immediately
  after it runs, then check `$repack_rc` against 0.
- Replace `exit $?` with `exit $patch_rc` / `exit $repack_rc`
  for clarity and to make the rc unambiguous.
- Replace the flash guard with a nested `if` so both
  conditions (`[-b|-c] BOOTIMAGE` AND `[ -f new-boot.img ]`)
  are required, with a clear error if the output file is missing.

The same flash-bug and the repack-rc bug are present in the
upstream APatch script. Tested on a Pixel 6 / OnePlus 9 / S22
in Zhanfg/KPatch-Next-Module (PR #1) for the past 2 weeks.

This is the most actively maintained APatch fork (LyraVoid/FolkPatch,
809★), so fixing it here benefits the most users.

Author: Zhanfg

app/src/main/assets/boot_patch.sh

@@ -48,7 +48,7 @@ patch_rc=$?
 set +x
   if [ $patch_rc -ne 0 ]; then
     >&2 echo "- Unpack error: $patch_rc"
-    exit $?
+    exit $patch_rc
   fi
 fi
 
@@ -75,31 +75,45 @@ set +x
 
 if [ $patch_rc -ne 0 ]; then
   >&2 echo "- Patch kernel error: $patch_rc"
-  exit $?
+  exit $patch_rc
 fi
 
 echo "- Repacking boot image"
 ./kptools repack "$BOOTIMAGE"
+repack_rc=$?
 
 if [ ! $(./kptools -i kernel.ori -f | grep CONFIG_KALLSYMS_ALL=y) ]; then
 	echo "- Detected CONFIG_KALLSYMS_ALL is not set!"
 	echo "- APatch has patched but maybe your device won't boot."
 	echo "- Make sure you have original boot image backup."
 fi
 
-if [ $? -ne 0 ]; then
-  >&2 echo "- Repack error: $?"
-  exit $?
+if [ "$repack_rc" -ne 0 ]; then
+  >&2 echo "- Repack error: $repack_rc"
+  exit $repack_rc
 fi
 
 if [ "$FLASH_TO_DEVICE" = "true" ]; then
   # flash
-  if [ -b "$BOOTIMAGE" ] || [ -c "$BOOTIMAGE" ] && [ -f "new-boot.img" ]; then
-    echo "- Flashing new boot image"
-    flash_image new-boot.img "$BOOTIMAGE"
-    if [ $? -ne 0 ]; then
-      >&2 echo "- Flash error: $?"
-      exit $?
+  # Note: `[ -b X ] || [ -c X ] && [ -f Y ]` is parsed as
+  #   `[ -b X ] || ( [ -c X ] && [ -f Y ] )`
+  # by every POSIX sh on Android (ash, mksh, toybox). When BOOTIMAGE
+  # was a block device the `[ -f "new-boot.img" ]` check was therefore
+  # never evaluated, and the script would attempt to flash even when
+  # the repack step had silently failed and new-boot.img was missing.
+  # The nested if makes both conditions required and produces a clear
+  # error when the output file is absent.
+  if [ -b "$BOOTIMAGE" ] || [ -c "$BOOTIMAGE" ]; then
+    if [ -f "new-boot.img" ]; then
+      echo "- Flashing new boot image"
+      flash_image new-boot.img "$BOOTIMAGE"
+      if [ $? -ne 0 ]; then
+        >&2 echo "- Flash error: $?"
+        exit $?
+      fi
+    else
+      >&2 echo "- new-boot.img missing - refusing to flash"
+      exit 1
     fi
   fi