bb: do not clobber the bash symlink

This gets confusing when working with both busybox and bash, as most of
our systems do.  If you install busybox after bash, such as by cd
tools/apps/busybox; make, then /bin/bash -> busybox.

It seems nice to have the bash symlink get covered by busybox, but then
again, on Akaros, the #!/bin/bash line doesn't get handled by the
kernel.  The shells just run the script internally, so they don't
handler the interpreter line.  Parlib doesn't even do it - it runs
"/bin/sh WHATEVER."

Signed-off-by: Barret Rhoden <brho@cs.berkeley.edu>

tools/apps/busybox/defconfig-1.17.3

@@ -926,9 +926,9 @@ CONFIG_ASH_EXPAND_PRMT=y
 CONFIG_FEATURE_SH_IS_ASH=y
 # CONFIG_FEATURE_SH_IS_HUSH is not set
 # CONFIG_FEATURE_SH_IS_NONE is not set
-CONFIG_FEATURE_BASH_IS_ASH=y
+# CONFIG_FEATURE_BASH_IS_ASH is not set
 # CONFIG_FEATURE_BASH_IS_HUSH is not set
-# CONFIG_FEATURE_BASH_IS_NONE is not set
+CONFIG_FEATURE_BASH_IS_NONE=y
 # CONFIG_LASH is not set
 # CONFIG_MSH is not set
 CONFIG_SH_MATH_SUPPORT=y