build: align SOURCE path for build system and SDK

build: align SOURCE path for build system and SDK

Building a package in the build system or the SDK results in different
values for the `SOURCE` property, it's either `packages/<package name>`
or `feeds/base/<package name>`. The reason is that the SDK handles
`openwrt.git` as an external feed called while the build system contains
the *base* packages directly.

Since packages created with either method are (ideally) the same (bit
for bit), align the content of SOURCE. To do so this commit creates a
symlink from `feeds/base` to `$(TOPDIR)/package` and adopts the SOURCE
when building from inside the build system.

Signed-off-by: Paul Spooren <mail@aparcar.org>