busybox: fix musl build issues

busybox: fix musl build issues

Musl doesn't implemented wmtp, and its utmp support is not sufficient
to build the latest versions of Busybox, so let's disable wtmp and
utmp support in Busybox for musl toolchains.

We re-use an existing define BUSYBOX_INTERNAL_SHADOW_PASSWORDS that
was used to do some musl-related tweaks to the Busybox configuration
around shadow password functions, and extend it to also do the
utmp/wtmp tweaks. In the process, we rename the define to
BUSYBOX_MUSL_TWEAKS.

Note that BUSYBOX_KCONFIG_FIXUP_CMDS continues to call
BUSYBOX_INTERNAL_SHADOW_PASSWORDS because there is another definition
of this macro under other conditions.

Fixes:

  http://autobuild.buildroot.org/results/b96/b96ffad13bca5e6fbf7ce79538aacbfe27b1eff3/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>