helper: add base64 encoding/decoding helpers from FreeBSD

These helpers are needed by the updated RISC-V target files.

Change-Id: I5aa9f4e58eb75e1c7a1e8e0e3961725e2a915ebb
Signed-off-by: Bernhard Rosenkränzer <bero@baylibre.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8895
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/helper/Makefile.am

@@ -3,6 +3,7 @@
 noinst_LTLIBRARIES += %D%/libhelper.la
 
 %C%_libhelper_la_SOURCES = \
+	%D%/base64.c \
 	%D%/binarybuffer.c \
 	%D%/options.c \
 	%D%/time_support_common.c \
@@ -18,6 +19,7 @@ noinst_LTLIBRARIES += %D%/libhelper.la
 	%D%/jim-nvp.c \
 	%D%/nvp.c \
 	%D%/align.h \
+	%D%/base64.h \
 	%D%/binarybuffer.h \
 	%D%/bitfield.h \
 	%D%/bits.h \