Revert "core/pkg-kconfig: use olddefconfig when available"

Revert "core/pkg-kconfig: use olddefconfig when available"

This reverts commit 638ea2d715950f1409475e01a37f25c3528df35e, since it
breaks the build of busybox in many configurations, with the following
issues:

/bin/bash: : command not found
make[1]: Entering directory `/home/test/autobuild/instance-0/output/build/busybox-1.24.2'
[...]
  GEN     miscutils/Config.in
make[2]: *** No rule to make target `olddefconfig'.  Stop.
make[1]: *** [olddefconfig] Error 2
make[1]: Leaving directory `/home/test/autobuild/instance-0/output/build/busybox-1.24.2'
make: *** [/home/test/autobuild/instance-0/output/build/busybox-1.24.2/.config] Error 1
make: Leaving directory `/home/test/autobuild/instance-0/buildroot'

See:

  http://autobuild.buildroot.org/results/48481bb3531927be35cda3df57d02f02cec709aa/

for example.

A minimal configuration with just Busybox doesn't exhibit the problem,
but adding libtirpc in the configuration is enough to trigger the
problem, so the following defconfig allows to reproduce the issue:

BR2_arm=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-arm-full-2016.02-3-g762b7c9.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_4_7=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_10=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_INET_RPC=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_PACKAGE_LIBTIRPC=y

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>