Currently the help for LOCAL_PATCH_DIR did not specify the tree layout
of custom patches directory. This commit adds such explanation.
For example, the bundled patches for GCC are placed under
packages/gcc/<gcc-version>, thus custom (local) GCC patches should be
placed under $LOCAL_PATCH_DIR/gcc/<gcc-version>.
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>