recovery: Allow custom bootloader msg offset in block misc

Squash of changes
  * Allow custom bootloader msg offset in block misc
    Change-Id: I6f3de1c2e3745c5535c3b95399abb8b2e2d01446

  * recovery: Move bldrmsg offset symbols to bootloader_message.cpp
    Change-Id: I2b902bcce7f5ca13472e0ac30ac01b4991294dbe

Adapted for Android.bp version of libbootloader_message

 * For android-11, these definitions are now used outside of
   the library itself, so keep them in the header, and export the
   necessary c flag to dependant modules

Change-Id: I5768660569d56b0d7c358f562e79483fd5f6db53