Config.in: Fixup after BR2_ELF2FLT got renamed to BR2_PACKAGE_HOST_ELF2FLT

The BR2_ELF2FLT option has been renamed to BR2_PACKAGE_HOST_ELF2FLT, thus
use the new name to fix dependency.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>