Disable ccache for embedded.o

Embedded image support uses .incbin in inline assembly to include binary
files.  The file dependency is not spotted by ccache when deciding
whether or not to rebuild embedded.o.  This results in builds that
contain an outdated version of the embedded image when ccache is used.

Reported-by: Tim 'Shaggy' Bielawa <tbielawa@jabber.org>
Reported-by: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>