local.conf: remove PREFERRED_PROVIDER for xserver-nodm-init

Remove not working as requested PREFERRED_PROVIDER_xserver-nodm-init_nitrogen6x.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
local.conf: remove not required macros

Remove not required macros MACHINE_KERNEL_PR = "r0" and
ANGSTROM_EGLIBC_VERSION = "2.16"

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
local.conf: mask entrance package

Mask bitbake package 'entrance' because this package also provides the opkg
packet 'xserver-nodm-init'. But the polo-image needs the original
'xserver-nodm-init' from bitbake package 'xserver-nodm-init'. If there are
two opkg packets that provide 'xserver-nodm-init' opkg installs the first.
And 'entrance' comes before 'xserver-nodm-init'. Therefore, 'entrance' must
never be built and 'entrance' ipk must not appear in the ipk installation
directory.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>