swupdate: Lua 5.1 / LuaJIT are not supported

swupdate: Lua 5.1 / LuaJIT are not supported

Before commit 87b6ac1478821351c92f7ca1c3154550e4713b28 support for Lua
was always disabled by the default config file:

```
CONFIG_LUA is not set
```

The commit removed this setting and Lua support is now enabled if a Lua
interpreter is enabled. As the swupdate build system uses pkg-config to
check for the lua library by default (LUAPKG="lua") this throws a lot of
undefined referenced in case LuaJIT is uses as Lua interpreter, e.g.:

```
corelib/lib.a(lua_interface.o): In function `l_info':
lua_interface.c:(.text.l_info+0x14): undefined reference to `luaL_checklstring'
```

In addition, since version 2017.07, Lua 5.1 really isn't supported any
more (before this version, it was only unsupported if the option
CONFIG_HANDLER_IN_LUA was set).  Therefore, remove support for Lua 5.1
and LuaJIT in the swupdate package for now until upstream might fix
this issue.

Reported upstream:
https://groups.google.com/forum/#!topic/swupdate/WAm8npAOd6o

Fixes:
http://autobuild.buildroot.net/results/df2/df2a71efe5af52d7b8721a355c49934b1be197a3/
http://autobuild.buildroot.net/results/400/4006225c8a47eb0b56399c83bd6d00406a0f62c2/
http://autobuild.buildroot.net/results/098/098f9d6cd905359adac4fb15e1d54c5022757325/
http://autobuild.buildroot.net/results/e5f/e5f6f99d96d9c661454335e7f931a03c3ae6a9e2/
http://autobuild.buildroot.net/results/f3a/f3a0abe8d5e35c85da40d20dab260c28506a0d4c/

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>