title: bitbake conditional inclusion of depends statement
body: <p>How do I include a depends line in a bitbake file with a condition ?
  I want something like below:</p>
  
  <pre><code>if (some env varible)
    DEPENDS += "recipe-1"
  else
    DEPENDS += "recipe-2'
  </code></pre>
  
  <p>I have tried below in the .bb file:</p>
  
  <pre><code>DEPENDS += "${@ 'recipe-2' if '${ENV_VAR}' else 'recipe-1'}"
  </code></pre>
  
  <p>Before that I exported ENV_VAR to BB_ENV_EXTRAWHITE</p>
  
  <pre><code>export BB_ENV_EXTRAWHITE="$BB_ENV_EXTRAWHITE ENV_VAR"
  </code></pre>
  
  <p>This is working only when ENV_VAR is set:</p>
  
  <pre><code>env ENV_VAR="value" bitbake test-recipe
  </code></pre>
  
  <p>if ENV_VAR is not set, it is throwing an error while parsing the bitbake DEPENDS line</p>
  
  <pre><code>ExpansionError: Failure expanding variable DEPENDS, expression was
  ${@ 'recipe-2' if '${ENV_VAR}' else 'recipe-1'}  
  which triggered exception SyntaxError: EOL while scanning string literal (DEPENDS, line 1)
  </code></pre>