For cases when we need to apply some patch over an existent package, we should use FILESEXTRAPATHS, which includes new directories into the searching algorithm, making the extra file visible to BitBake, as shown:

FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}-${PV}:"  
SRC_URI += "file://mypatch.patch" 

In the preceding example, the current directory / package version (${THISDIR}/${PN}-${PV}:) is included in the directories list used for file searching. The use of the _prepend operator is important as it guarantees that our provided file is used, even if a file with the same name is added in the lower priority layers in future.

BitBake assumes that every file with a .patch extension is a patch and applies it accordingly.