elf2flt: fix after conversion of binutils to package infrastructure

When binutils was converted to the package infrastructure (commit
009407e6), the variable that elf2flt uses to find the binutils
libraries disappeared.  So use HOST_BINUTILS_DIR instead of
BINUTILS_DIR1.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>