IMAGEMB settings

[peter eisch <peterbokunet%hotmail.com@localhost>]

Hi,

In the install-image and live-image (as it uses install-image) build targets 
the common Makefile sets IMAGEMB to be 2048.  In the i386 platform it sets this 
down to 512 (and 550 on amd64).  Is there a reason that these platforms are 
unable to use the default 2048?  More importantly is that the value can't be 
overridden as it's defined with "IMAGEMB=512" instead of "IMAGEMB?=512".

In my build I'd also like to also augment SPEC_EXTRA too.  Is there a mechanism 
for extending that too?

Thanks,

peter