nativesdk: ensure features don't get backfilled

nativesdk: ensure features don't get backfilled

nativesdk aims to run in a slightly different environment than the
target build, so it resets MACHINE_FEATURES and filters DISTRO_FEATURES
with DISTRO_FEATURES_NATIVESDK.

However, feature backfill happens _after_ these operations:

 $ bitbake-getvar -r nativesdk-glib-2.0 MACHINE_FEATURES
 #
 # $MACHINE_FEATURES [5 operations]
 #   set /home/ross/Yocto/poky/meta/conf/machine/include/qemu.inc:14
 #     "alsa bluetooth usbgadget screen vfat"
 #   set /home/ross/Yocto/poky/meta/conf/documentation.conf:284
 #     [doc] "Specifies the list of hardware features the MACHINE supports."
 #   set? /home/ross/Yocto/poky/meta/conf/bitbake.conf:893
 #     ""
 #   set /home/ross/Yocto/poky/meta/classes-recipe/nativesdk.bbclass:18
 #     ""
 #   append utils.py:132 [features_backfill]
 #     " rtc qemu-usermode"
 # pre-expansion value:
 #   " rtc qemu-usermode"
 MACHINE_FEATURES=" rtc qemu-usermode"

This is not intentional nor desired as the target machine features are
unrelated to the nativesdk environment.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Link: https://lore.kernel.org/r/20231221170159.1995650-1-ross.burton@arm.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>