[U-Boot] Improve default "make" targets (ALL-y)

[Loïc Minier]

        Hey

 For the Debian package, we currently maintain lists like:
dbau1100        u-boot.img
dockstar        u-boot.kwb
eNET            u-boot.bin
efikamx         u-boot.imx
omap4_panda     MLO u-boot.bin
[...]

 for two reasons:
 - because we need to call make u-boot.kwb explicitly to get it built
 - to list the files from the obj-dir that we actually want in the .deb

 it would be nicer for the packaging, but I guess also for u-boot
 maintainers, if these files were just generated by default when one
 calls "make".

 The subsequent patches will generate u-boot.imx and u-boot.kwb when
 it's obvious that these are desired.

 However, I didn't manage to find hints in the u-boot tree on when
 "u-boot.img" should be generated, and when u-boot.bin is enough;
 apparently one can mkimage -T firmware for any board config.  Ideas
 welcome on how to detect these.

   Thanks,

-- 
Loïc Minier