elflink: Fix compiler warnings by including core-elf.h

elflink: Fix compiler warnings by including core-elf.h

Numerous files need to include core-elf.h to get the definition of
'enum kernel_type'.

In file included from mytest.c:19:0:
menu.h: At top level:
menu.h:206:40: warning: ‘enum kernel_type’ declared inside parameter list
menu.h:206:40: warning: its scope is only this definition or declaration, which is probably not what you want

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