base64: remove duplicate ‘const’ declaration specifier ...

By cleaning up the code the compiler does not emit the following warning
anymore:

    src/base64.c:15:19: warning: duplicate ‘const’ declaration specifier [-Wduplicate-decl-specifier]
     static const char const uuenc_tbl_base64[65 + 1] = {
                       ^~~~~

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>

base64: remove unused variable ...

The variable seems to be a leftover from importing the function from
busybox-1.23.2 and by cleaning up the code the compiler does not emit the
following warning anymore:

    src/base64.c:34:14: warning: variable ‘src_tail’ set but not used [-Wunused-but-set-variable]
      const char *src_tail;
                  ^~~~~~~~

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>

base64: import from barebox ...

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>