config.mk: specify the exact path to standalone linker script

We want to change the build system to include config.mk
only from ./Makefile and spl/Makefile.
We must prepare for that in this commit.

$(src) is a moving target and not handy for our purpose.
We must replace it with a fixed path.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>