scripts/feeds: Reuse TOPDIR if defined in environment

scripts/feeds: Reuse TOPDIR if defined in environment

The feeds script sets value of TOPDIR in a way that is inconsistent
with how toplevel Makefile sets it. The inconsistency manifests when I
use a "build directory" with symlinks to LEDE source (see below).

When make is invoked in such a directory, make's TOPDIR variable is
set to that directory, whereas scripts/feeds sets TOPDIR to the top of
LEDE source, which results in creating feeds directory inside the LEDE
source instead of in the build directory.

This patch changes the script so that it reuses the TOPDIR value form
the environment if it exists. The result is that 'make
package/symlinks' correctly fetches feeds to the build directory
instead in the source.

I use the following commands to create the build directory:

    ln -s $SRC/config config
    ln -s $SRC/Config.in Config.in
    ln -s $SRC/feeds.conf.default feeds.conf.default
    ln -s $SRC/include include
    ln -s $SRC/Makefile Makefile
    mkdir package
    ln -s $SRC/package/base-files package/base-files
    ln -s $SRC/package/boot package/boot
    ln -s $SRC/package/devel package/devel
    ln -s $SRC/package/firmware package/firmware
    ln -s $SRC/package/kernel package/kernel
    ln -s $SRC/package/libs package/libs
    ln -s $SRC/package/Makefile package/Makefile
    ln -s $SRC/package/network package/network
    ln -s $SRC/package/system package/system
    ln -s $SRC/package/utils package/utils
    ln -s $SRC/rules.mk rules.mk
    ln -s $SRC/scripts scripts
    ln -s $SRC/target target
    ln -s $SRC/toolchain toolchain
    ln -s $SRC/tools tools

This allows me to easily test changes in LEDE on multiple targets.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>