um: Fix build error and kconfig for i386

Fix build error by generating elfcore.o only when ELF_CORE (depending on
COREDUMP) is selected:

arch/x86/um/built-in.o: In function `elf_core_write_extra_phdrs':
(.text+0x3e62): undefined reference to `dump_emit'
arch/x86/um/built-in.o: In function `elf_core_write_extra_data':
(.text+0x3eef): undefined reference to `dump_emit'

Fixes: 5d2acfc7 ("kconfig: make allnoconfig disable options behind EMBEDDED and EXPERT")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

arch/x86/um/Makefile

@@ -17,7 +17,7 @@ obj-y = bug.o bugs_$(BITS).o delay.o fault.o ksyms.o ldt.o \
 ifeq ($(CONFIG_X86_32),y)
 
 obj-y += checksum_32.o
-obj-$(CONFIG_BINFMT_ELF) += elfcore.o
+obj-$(CONFIG_ELF_CORE) += elfcore.o
 
 subarch-y = ../lib/string_32.o ../lib/atomic64_32.o ../lib/atomic64_cx8_32.o
 subarch-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += ../lib/rwsem.o