unix: Deprecate support for GNU Readline (MICROPY_USE_READLINE=2).

MicroPython own readline implementation is superior now by providing
automatic indentation and completion (completion for GNU Readline was
never implemented). MICROPY_USE_READLINE=2 also wasn't build for a long
time and probably broken.

If GNU Readline is still beneficial for some cases, it can be achieved
with external wrappers like "rlwrap" (there will be the same level of
functionality, as again, there never was deep integration, like completion
support).

unix/Makefile

@@ -85,12 +85,6 @@ INC +=  -I../lib/mp-readline
 CFLAGS_MOD += -DMICROPY_USE_READLINE=1
 LIB_SRC_C_EXTRA += mp-readline/readline.c
 endif
-ifeq ($(MICROPY_USE_READLINE),2)
-CFLAGS_MOD += -DMICROPY_USE_READLINE=2
-LDFLAGS_MOD += -lreadline
-# the following is needed for BSD
-#LDFLAGS_MOD += -ltermcap
-endif
 ifeq ($(MICROPY_PY_TIME),1)
 CFLAGS_MOD += -DMICROPY_PY_TIME=1
 SRC_MOD += modtime.c

unix/mpconfigport.mk

@@ -6,7 +6,6 @@ MICROPY_FORCE_32BIT = 0
 # This variable can take the following values:
 #  0 - no readline, just simple input
 #  1 - use MicroPython version of readline
-#  2 - use GNU readline (causes binary to be licensed under GPL)
 MICROPY_USE_READLINE = 1
 
 # Whether to enable FatFs VFS