build: Define _BOOT_MODE once and for all in ArchitectureRules+BootRules.

Previously it was spread all around the tree, and was not defined
consistently for all boot objects (there were a number of boot modules
which did not define it, but did include headers which checked for it.)

Now, as it is handled in SetupBoot which is invoked for all boot objects,
it will be applied consistently throughout. We can thus drop the manual
additions of it from all Jamfiles.

arm64: Enable DTB handling, Introduce LINFlex UART driver

Change-Id: Ib643545271700e6ff4a4037d0e797355194927e7
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5149
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: David Karoly <karolyd577@gmail.com>
Reviewed-by: Fredrik Holmqvist <fredrik.holmqvist@gmail.com>

ARM64 Jamfiles, missed in previous commit

* Use generic c versions of memset and memcpy
Wwrite fast asm for that later, therefore no need for arch_string.S
* Disable FDT and DTB until ARM64 is ready
(WIP: do it later)
* use ELF64

arm64: Add more random scripts to get stuff to compile

Signed-off-by: Jaroslaw Pelczar <jarek@jpelczar.com>
Change-Id: Ie043af5b7471f626a1ffe100848151c832dcc439
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1853
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>