fix -elf2flt usage for bfin-uclinux

There are multiple problems with the elf2flt usage in the configure script:
 - clobbers LDFLAGS
 - is used for all targets so breaks non-FLAT targets
 - is only used for Blackfin FLAT targets
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

configure.ac

@@ -188,8 +188,10 @@ fi])
 AC_ARG_ENABLE(blackfin-asm, [  --enable-blackfin-asm   Make use of Blackfin assembly optimizations],
 [if test "$enableval" = yes; then
   AC_DEFINE([BFIN_ASM], , [Make use of Blackfin assembly optimizations])
-  LDFLAGS="-Wl,-elf2flt=-s100000"
 fi])
+case $host_os in
+  uclinux) LDFLAGS="-Wl,-elf2flt=-s100000 $LDFLAGS";;
+esac
 
 AC_ARG_ENABLE(fixed-point-debug, [  --enable-fixed-point-debug  Debug fixed-point implementation],
 [if test "$enableval" = yes; then