cc2538: Fix library linkage

If a project needs to use some libraries at link stage, then the corresponding
linker options (e.g. '-lm') have to be passed after any .o file depending on
these libraries. Hence, since LDFLAGS is the Makefile variable that would be
used to add such options, move it to the correct location when invoking $(LD).

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge branch 'cc2538-lib-link' into advansee

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>