nativesdk.bbclass: support nativesdk to override with the PACKAGES_DYNAMIC statement

While compiling nativesdk-mtools, there was failure:
...
Nothing PROVIDES 'nativesdk-glibc-gconv-ibm850'. Close matches:
...
This patch supports nativesdk to override with the PACKAGES_DYNAMIC statement

[YOCTO #5623]
Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>