bbox: musl versus uclibc

[Denys Vlasenko <vda.linux@googlemail.com>]

As uclibc is increasingly aging, I am finally forced
to switch to musl: I'm bitten by a nasty bug in
getopt() - hush is using it in a slightly unusual way,
which uclibc does not expect.

I built a toolchain using
    https://github.com/richfelker/musl-cross-make
(Rich, is this the thing I should be using?)
and it worked with no issues at all.

(I can probably only wish for the README
to also mention how to make this a _static_
toolchain... I have a box with 32-bit userspace,
would be awesome to be able to copy this fresh
64-bit toolchain to it and have it working).

Then I built busybox. Impressions:

Only a few options did not build:
EXTRA_COMPAT and FEATURE_VI_REGEX_SEARCH
failed because they need GNU regexp extensions.
FEATURE_MOUNT_NFS and FEATURE_INETD_RPC do not build
because they need rpc/rpc.h.
Not complaining, since them being in libc was a mistake
in the first place.

Now, the good news - musl has smaller data!
6695 bytes versus 7129 bytes for uclibc:

   text  data   bss     dec    hex filename
 894902   465  6664  902031  dc38f busybox.uclibc
 912538   563  6132  919233  e06c1 busybox.musl

Whee!