U-Boot: ARM: Add a linux entry point to asm shell code

While the NetBSD entry point is handy as we can use a single uImage
with all 3 blobs, it bypasses U-Boot's own patching of the FDT since
it's not visible to it, so we won't get the RAM size and other things
through it.
U-Boot: generate a separate uImage for the boot tgz as well

We need this when using the linux entry point.
ARM: overo: Use the linux entry point version of the loader

And load the boot tgz and FDT separately from the SD card.

Hopefully U-Boot will now report available RAM size.
ARM: Check for RAM size in FDT

We skip the check when we already have ranges inserted,
like from the raspberry Pi start code, and we fall back to
32MB at SDRAM_BASE is not found.
U-Boot: drop the bind on the flash image action