bash: fix build with binutils 2.24.90

binutils 2.24.90 fails to recognize the -l:path/to/lib.a syntax, which
was used to point explicitly to the in-tree libreadline.a using the
READLINE_LIB option passed to the bash build. this is needed for
cross-compilation due to -L path priority issues

a bug has been filed to upstream at
https://sourceware.org/bugzilla/show_bug.cgi?id=17532

the error was
"/bin/ld: cannot find -l:lib/readline/libreadline.a"