package/mkpimage: define extract step to allow patching

package/mkpimage: define extract step to allow patching

As recently discussed on the list:
http://lists.busybox.net/pipermail/buildroot/2021-April/308373.html

Packages like mkpimage where the source code is included in the Buildroot
tree, currently doesn't use an extract step (which also skips the patch
step), so they cannot be patched by a patch in BR2_GLOBAL_PATCH_DIR (or the
pkgdir), which is a bit confusing.

As a fix, add a simple extract step that copies the source file to the build
directory, so the patch step gets run as well.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>