rpm: Fix the following error when run nativesdk-rpm in nativesdk environment.

rpm: Fix the following error when run nativesdk-rpm in nativesdk environment.

  package busybox-inittab-1.36.1-r0.ubinux_x86_64 is intended for a different architecture
  package busybox-udhcpc-1.36.1-r0.core2_64 is intended for a different architecture
  package ldconfig-2.39+git0+312e159626-r0.core2_64 is intended for a different architecture
  ......

CMAKE_INSTALL_SYSCONFDIR is necessary. If not set
CMAKE_INSTALL_SYSCONFDIR, nativesdk-rpm will use the default setting:
/usr/local/oe-sdk-hardcoded-buildpath/sysroots/x86_64-ubinuxsdk-linux/etc to find rpmrc file.

Signed-off-by: Lei Maohui <leimaohui@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>