base-files: fix build for /sbin/pkg_check

Setting CONFIG_IPK_FILES_CHECKSUMS=y causes sha256 checksum files to be
included with the packages to check for corruption. This commit fixes two
issues:
- /sbin/pkg_check was being removed incorrectly if IPK_FILES_CHECKSUMS=y
- checksums were being saved in the wrong file

Signed-off-by: Xu Wang <xwang1498@gmx.com>

include/package-ipkg.mk

@@ -207,7 +207,7 @@ $(_endef)
 	(cd $$(IDIR_$(1)); \
 		( \
 			find . -type f \! -path ./CONTROL/\* -exec sha256sum \{\} \; 2> /dev/null | \
-			sed 's|\([[:blank:]]\)\./|\1/|' > $$(IDIR_$(1))/CONTROL/files-sha256 \
+			sed 's|\([[:blank:]]\)\./|\1/|' > $$(IDIR_$(1))/CONTROL/files-sha256sum \
 		) || true \
 	)
     endif