toolchain: linker options with a $ sign are not supported

As reported in bug #7172 [0], setting BR2_TARGET_LDFLAGS to a value
containing a $ sign can lead to unexpected results.

This is because it is very hard to know when the $ sign gets evaluated:
  - in the Buildroot-level make
  - in the shell called by the Buildroot-level make
  - in the package's own build-system, either at configure time, in the
    Makefile, in a shell in the Makefile...

So, it is very difficult to know how much escaping that would need.

A proposal is to use a shell variable to pass such values unmolested.
But it is not that simple either, since it still contains a $ sign, and
there is not much certainty as to when it would be evaluated.

Instead, just document this limitation, both in the help text for
BR2_TARGET_LDFLAGS, and in the known-issues section in the manual.

Does not really fix #7172, but at least the limitation is documented.

[0] https://bugs.buildroot.org/show_bug.cgi?id=7172



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Mike Zick <minimod@morethan.org>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>