builder: add env var to apply extra patches

Example:
S1B_EXTRA_KERNEL_PATCH_URLS=https://github.com/kinvolk/linux/commit/5059f528a006dc635c9951b9f586ad6932860fad.patch S1B_KERNEL_VERSION=4.10.6 ./builder

builder

@@ -86,7 +86,14 @@ test -f "${kernel_bzimage}" ||
   cd "${kernel_source_dir}"
   curl -LsS "${kernel_reboot_patch_url}" -O
   # TODO(schu) fails when patch was applied already
-  patch --silent -p1 < *.patch
+  patch --silent -p1 < $(basename "${kernel_reboot_patch_url}")
+  for patch_url in ${S1B_EXTRA_KERNEL_PATCH_URLS} ; do
+    curl -LsS "${patch_url}" -O
+    patch --silent -p1 < $(basename "${patch_url}")
+  done
+  for patch_file in ${S1B_EXTRA_KERNEL_PATCH_FILES} ; do
+    patch --silent -p1 < "${patch_file}"
+  done
   ${mk} bzImage
 )
 

builder-config

@@ -24,3 +24,7 @@ S1B_BUILD_DIR="${S1B_BUILD_DIR:-${DIR}/build/${S1B_KERNEL_VERSION}}"
 
 # The directory to store the aci contents (manifest, rootfs) in
 S1B_ACI_DIR="${S1B_ACI_DIR:-${DIR}/aci/${S1B_KERNEL_VERSION}}"
+
+# Optional extra kernel patches to apply
+S1B_EXTRA_KERNEL_PATCH_FILES="${S1B_EXTRA_KERNEL_PATCH_FILES:-}"
+S1B_EXTRA_KERNEL_PATCH_URLS="${S1B_EXTRA_KERNEL_PATCH_URLS:-}"