Found what I was asking for with:

bitbake -e <package>

going through the output, there are mentions of patch_do_patch().

I've traced it to meta/classes/patch.bbclass.

However, trying to execute just the patching for the recipe doesn't work.

build⟫ bitbake -c patch u-boot-fw-utils-senic

...

ERROR: Task do_patch does not exist for target u-boot-fw-utils-senic (/home/alan/senic-os-update/oe/../meta-senic/recipes-bsp/u-boot/u-boot-fw-utils-senic_2017.03.bb:do_patch). Close matches:

  do_fetch

  do_unpack

ERROR: Command execution failed: 1