Merge pull request #190 from ranma/fix-bit-linking

Fix declaration of tx_buf_empty bit var

__sbit is for SFRs, for regular data __bit should be used. Otherwise the variables is not correctly declared in the BSEG section, leading to bit temporaries being allocated in the same location.

When this causes the tx_buf_empty to be overwritten to 0, then the next write_char will hang forever.

With this change the bit is properly declared in BSEG for the linker, which in my testing resolves the overlap issue:
```
;--------------------------------------------------------
; bit data
;--------------------------------------------------------
        .area BSEG    (BIT)
_tx_buf_empty::
        .ds 1
```

Only the vlan and mtu commands call atoi_short, which has a local bit _atoi_short_sloc0_1_0 which is the first bit in the BSEG, and assigning to that one corrupted the tx_buf_empty bit.
Though this is also a compiler optimization-failure, the _atoi_short_sloc0_1_0 value isn't even used after assignment, AFAICS it is a dead store.

Its from the isnumber inline:
```
[...]
;--------------------------------------------------------
; bit data
;--------------------------------------------------------
        .area BSEG    (BIT)
_atoi_short_sloc0_1_0:
        .ds 1
_parse_ip_sloc0_1_0:
        .ds 1
[...]
;       cmd_parser.c:85: l -= '0';
        mov     r3,a  
        add     a,#0xd0
;       cmd_parser.c:86: return (l <= ('9'-'0'));
        add     a,#0xff - 0x09
        cpl     c
;       cmd_parser.c:172: while (isnumber(cmd_buffer[idx])) {
        mov     _atoi_short_sloc0_1_0,c
        jnc     00103$
[...]
```

rtlplayground.c

@@ -137,7 +137,7 @@ __xdata char sfp_module_serial[2][17];
 __xdata uint8_t sfp_options[2];
 __xdata bool button_last;
 __xdata uint8_t button_sec_counter_last;
-__sbit tx_buf_empty;
+volatile __bit tx_buf_empty;
 
 #define ETHERTYPE_OFFSET (12 + VLAN_TAG_SIZE + RTL_TAG_SIZE)