Harden GRUB recipe with error guards and host-tool verification

Add || exit 1 to all critical build steps (mkdir, cd, touch, configure,
make, grub-mkimage) so failures surface immediately instead of silently
continuing. Verify gcc/make/bison/flex are present before starting the
build. Update grub.cfg help text to reference fat_tool.py instead of
unavailable mtools.

local/recipes/core/grub/recipe.toml

@@ -26,12 +26,20 @@ script = """
 unset CC CXX CPP LD AR NM RANLIB OBJCOPY STRIP PKG_CONFIG
 unset CFLAGS CXXFLAGS CPPFLAGS LDFLAGS
 
+# Verify host tools are available before starting the build.
+for tool in gcc make bison flex; do
+    if ! command -v "${tool}" &>/dev/null; then
+        echo "ERROR: Required host tool '${tool}' not found" >&2
+        exit 1
+    fi
+done
+
 # Out-of-tree build (GRUB requires this for some configurations)
-mkdir -p "${COOKBOOK_BUILD}/grub-build"
-cd "${COOKBOOK_BUILD}/grub-build"
+mkdir -p "${COOKBOOK_BUILD}/grub-build" || exit 1
+cd "${COOKBOOK_BUILD}/grub-build" || exit 1
 
 # GRUB release tarballs miss extra_deps.lst (normally generated by autogen.sh)
-touch "${COOKBOOK_SOURCE}/grub-core/extra_deps.lst"
+touch "${COOKBOOK_SOURCE}/grub-core/extra_deps.lst" || exit 1
 
 # Configure for host, targeting x86_64 EFI platform output.
 # --target=x86_64 means "produce boot images for x86_64"
@@ -48,10 +56,10 @@ echo "Configuring GRUB for x86_64 EFI..."
     --disable-grub-mount \
     --disable-device-mapper \
     --disable-libzfs \
-    --prefix="${COOKBOOK_STAGE}/usr"
+    --prefix="${COOKBOOK_STAGE}/usr" || exit 1
 
 echo "Building GRUB..."
-make -j "${COOKBOOK_MAKE_JOBS}"
+make -j "${COOKBOOK_MAKE_JOBS}" || exit 1
 
 # Create output directory
 mkdir -p "${COOKBOOK_STAGE}/usr/lib/boot"
@@ -101,7 +109,7 @@ echo "Creating standalone GRUB EFI image..."
     ls \
     cat \
     halt \
-    reboot
+    reboot || exit 1
 
 # Verify output
 if [ ! -f "${COOKBOOK_STAGE}/usr/lib/boot/grub.efi" ]; then