This removes lines that set PKG_BUILD_DIR when the set value is no
different from the default value.

Specifically, the line is removed if the assigned value is:

* $(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-$(PKG_VERSION)

  The default PKG_BUILD_DIR was updated[1] to incorporate BUILD_VARIANT
  if it is set, so now this is identical to the default value.

* $(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_SOURCE_SUBDIR)

  if PKG_SOURCE_SUBDIR is set to $(PKG_NAME)-$(PKG_VERSION), making it
  the same as the previous case

* $(BUILD_DIR)/$(PKG_NAME)-$(PKG_VERSION)

  This is the same as the default PKG_BUILD_DIR when there is no
  BUILD_VARIANT.

* $(BUILD_DIR)/[name]-$(PKG_VERSION)

  where [name] is a string that is identical to PKG_NAME

[1]: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=e545fac8d968864a965edb9e50c6f90940b0a6c9

Signed-off-by: Jeffery To <jeffery.to@gmail.com>