Add defmt feature gate

[HaoboGu]

Adapted from #211

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
124252	    116	  41308	 165676	  2872c	rmk-nrf52840

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.1% +2.22Ki  [ = ]       0    .debug_loc
  +0.1% +1.63Ki  [ = ]       0    .debug_str
  +0.1%    +820  [ = ]       0    .debug_info
  +0.3%    +432  [ = ]       0    .strtab
  +0.5%     +80  [ = ]       0    .debug_aranges
  +0.0%     +32  [ = ]       0    .debug_ranges
  +0.1%     +32  [ = ]       0    .symtab
  +0.6%      +3  [ = ]       0    .defmt
  -2.9%      -2  [ = ]       0    [Unmapped]
  -0.1%      -6  [ = ]       0    .debug_abbrev
  -0.2%     -32  [ = ]       0    .debug_frame
  -1.1%    -184  -1.1%    -184    .rodata
  -0.1%    -187  [ = ]       0    .debug_line
  -0.2%    -200  -0.2%    -200    .text
  +0.2% +4.62Ki  -0.2%    -384    TOTAL

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
124252	    116	  41308	 165676	  2872c	rmk-nrf52840

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.1% +2.22Ki  [ = ]       0    .debug_loc
  +0.1% +1.63Ki  [ = ]       0    .debug_str
  +0.1%    +820  [ = ]       0    .debug_info
  +0.3%    +432  [ = ]       0    .strtab
  +0.5%     +80  [ = ]       0    .debug_aranges
  +0.0%     +32  [ = ]       0    .debug_ranges
  +0.1%     +32  [ = ]       0    .symtab
  +0.6%      +3  [ = ]       0    .defmt
  -2.9%      -2  [ = ]       0    [Unmapped]
  -0.1%      -6  [ = ]       0    .debug_abbrev
  -0.2%     -32  [ = ]       0    .debug_frame
  -1.1%    -184  -1.1%    -184    .rodata
  -0.1%    -187  [ = ]       0    .debug_line
  -0.2%    -200  -0.2%    -200    .text
  +0.2% +4.62Ki  -0.2%    -384    TOTAL

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
108108	    116	  41308	 149532	  2481c	rmk-nrf52840

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.8%    +122  [ = ]       0    .debug_abbrev
  +6.8%      +3  [ = ]       0    [Unmapped]
 -42.7%    -178  [ = ]       0    .defmt
  -2.1%    -352  -2.1%    -352    .rodata
  -4.5%    -788  [ = ]       0    .debug_frame
  -2.2% -2.22Ki  [ = ]       0    .debug_ranges
 -21.4% -3.41Ki  [ = ]       0    .debug_aranges
  -2.1% -4.15Ki  [ = ]       0    .debug_loc
 -11.7% -4.23Ki  [ = ]       0    .symtab
  -6.1% -8.34Ki  [ = ]       0    .debug_line
 -13.6% -14.2Ki -13.6% -14.2Ki    .text
  -2.9% -22.2Ki  [ = ]       0    .debug_info
 -28.2% -33.5Ki  [ = ]       0    .strtab
  -7.4% -87.7Ki  [ = ]       0    .debug_str
  -6.7%  -181Ki  -9.0% -14.5Ki    TOTAL

[github-actions]

PR build size

  text	   data	    bss	    dec	    hex	filename
124268	    116	  41308	 165692	  2873c	rmk-nrf52840

Diff

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.1% +2.22Ki  [ = ]       0    .debug_loc
  +0.1% +1.66Ki  [ = ]       0    .debug_str
  +0.1%    +779  [ = ]       0    .debug_info
  +0.4%    +474  [ = ]       0    .strtab
  +0.5%     +88  [ = ]       0    .debug_aranges
  +0.2%     +64  [ = ]       0    .symtab
  +0.0%     +40  [ = ]       0    .debug_ranges
  +0.6%      +3  [ = ]       0    .defmt
  -0.1%      -6  [ = ]       0    .debug_abbrev
  -0.1%     -16  [ = ]       0    .debug_frame
 -29.4%     -20  [ = ]       0    [Unmapped]
  -0.1%    -183  [ = ]       0    .debug_line
  -1.1%    -184  -1.1%    -184    .rodata
  -0.2%    -184  -0.2%    -184    .text
  +0.2% +4.71Ki  -0.2%    -368    TOTAL