u-boot/arm: simplify device tree handling

By updating $fdt_addr_r we can simplify boot-arm.scr.txt.

Now we can explicitly pass the device tree address to bootefi instead of
relying on a fallback inside U-Boot.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Change-Id: Ifceb574aca2814605a2e4e58e9187773817c5df2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2970
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

u-boot/arm: Break script out of Jamfile. Add EFI support.

* boot.scr was growing past Jam MAXLINE and was causing segfaults
* Break out to tree, data/boot could be used for other arches
* boot.scr is getting a bit complex, but supports the old
u-boot haiku_loader, as well as u-boot starting our EFI loader.

Change-Id: I93a3167ab0bc78747c1d70fedd08595e9ac89662
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2963
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>