build: allow file modes per binary package

build: allow file modes per binary package

Currently the global variable PKG_FILE_MODES is used for all ipkg
creations. This works for Makefiles which output a single package, or
variants of a single package.

But if a Makefile outputs multiple packages that each contain different
files, setting PKG_FILE_MODES causes build failure when any of the files
in the variable do not exist in the folder that is currently being
packaged.

Example:

/openwrt/staging_dir/host/bin/fakeroot -l /openwrt/staging_dir/host/lib/libfakeroot.so -f /openwrt/staging_dir/host/bin/faked /openwrt/scripts/ipkg-build -m "/usr/lib/mariadb/plugin/auth_pam_tool_dir:root:376:0750" /openwrt/build_dir/target-mips_24kc_musl/mariadb-10.4.13/ipkg-mips_24kc/mariadb-server-plugin-disks /openwrt/bin/packages/mips_24kc/packages
+chown: cannot access '/openwrt/build_dir/target-mips_24kc_musl/mariadb-10.4.13/ipkg-mips_24kc/mariadb-server-plugin-disks//usr/lib/mariadb/plugin/auth_pam_tool_dir': No such file or directory

This commit changes the file mode handling a bit. The file mode can now
be set either globally via PKG_FILE_MODES (no behavior change) or on a
per-package basis via FILE_MODES. This way specific file modes can be
used for any particular package.

This behavior is already used for other OpenWrt variables, hence it is
familiar:

PKG_MAINTAINER vs MAINTAINER
PKG_SOURCE_SUBDIR vs SUBDIR
PKG_LICENSE vs LICENSE
...

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>