elf_gen_dep.sh: Prioritise symbols exported by core

elf_gen_dep.sh: Prioritise symbols exported by core

It's possible for multiple modules to export the same symbol. We
always favour the symbols exported from core/ because this means that
modules will have less runtime dependencies.

For example, some symbols required by ldlinux.c32 are exported by both
core/ and libmenu.c32. Because we don't want ldlinux.c32 to have any
dependencies, we need to make sure we resolve to the symbol exported
by core/.

Note that now the order in which we extract global symbols from ELF
files in elf_gen_dep.sh is important, because this dictates our order
of preference and hence core/isolinux.elf and core/pxelinux.elf need
to come first.

Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>