grub: Update recipe and patches

The builddir/sourcedir split changes in 1.7 require a few build
changes e.g. -I. no longer does what it implies, and top_srcdir was
misused in a couple places.

This also replaces grub's clanton.patch with the one from the latest
galileo 1.0.1 update, which is what I used to verify booting from
flash and is therefore most recently verified to work.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
meta-micro-galileo: Add image-micro-spi

This image simply creates all the artifacts needed for the galileo SPI
flash image creation tool.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
meta-micro-galileo: Add SPI flash instructions to README

Add detailed instructions for building and booting a new image from
SPI flash.

Also, refactor the sections a bit.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>