Add support for SDCARD image compression

Compression method to apply to SDCARD after it has been created. 
Supported compression formats are "gzip", "bzip2" or "xz". 
The original .sdcard file is kept and a new compressed file is created if one of these compression
formats is chosen. 
If SDCARD_COMPRESSION is set to any other value it is silently ignored.
SDCARD_COMPRESSION ?= variable can be set in local.conf