Add GET WIFISCAN and base64 WiFi credential commands #262

[unlogisch04]

The PR #262 has some issues for me.

1. On esp8266(not tested on EP32) the get wifiscan command does not work as expected (when the tracker finds no network the wifiscan fails, see comment on PR
2. @nekomona In the base64_decode_chars is that function save for bufferoverflow? output string can easy be shorter than input string?

[unlogisch04]

The tracker crashes if the b64 is too long

[INFO ] [SerialCommands] Sensor[1]: Unknown (0.000 0.000 0.000 1.000) is working: false, had data: false
set bwifi YWxhc2Rob2dpaGF3b2xnbnlrbHNudmZvaWhhZXNvZ25rbGphc2RsZmpvaWhqYWxuYXNsZGdhZGtsc2hnb2Fp

--------------- CUT HERE FOR EXCEPTION DECODER ---------------

Exception (28):
epc1=0x40236a98 epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000

>>>stack>>>

ctx: cont
sp: 3ffffd90 end: 3fffffd0 offset: 0150
3ffffee0:  3fff1ebc 3fff1db3 00000000 40212092
3ffffef0:  4022609c 4022b68c 3f3504f3 3fff3608
3fffff00:  fffff1b8 3fff364c 3fff3608 4020c024
3fffff10:  bc897638 3d34b45d be3a205d 0000005e  
3fffff20:  3fff1da9 3fff20ec 3fff1d9c 4022638c
3fffff30:  616c610a 6f686473 61686967 676c6f77
3fffff40:  6c6b796e 66766e73 6168696f 676f7365  
3fffff50:  6a6c6b6e 6c647361 696f6a66 6c616a68
3fffff60:  6c73616e 64616764 68736c6b 69616f67
3fffff70:  3fff1e00 3fff1eac 3fff1d9c 40212889
3fffff80:  00000000 3fff1aac 3fff1aa0 3fff2264
3fffff90:  00000000 3fff1d78 3fff1d78 402090ac
3fffffa0:  3fffdad0 00000000 3fff2234 3fff2264
3fffffb0:  3fffdad0 00000000 3fff2234 40224554  
3fffffc0:  feefeffe feefeffe 3fffdab0 4010155d
<<<stack<<<

--------------- CUT HERE FOR EXCEPTION DECODER ---------------

 ets Jan  8 2013,rst cause:2, boot mode:(3,6)