vexpress64: get rid of CONFIG_SYS_EXTRA_OPTIONS

The Versatile Express ARMv8 semihosted FVP platform is still
using the legacy CONFIG_SYS_EXTRA_OPTIONS method to configure
some compile-time flags. Get rid of this and create a Kconfig
entry for the FVP model, and a selectable bool for the
semihosting library.

The FVP subboard is now modeled as a target choice so we can
eventually choose between different ARMv8 versatile express
boards (FVP, base model, Juno...) this way. All dependent
symbols are updated to reflect this.

The 64bit Versatile Express board symbols are renamed
VEXPRESS64 so we have some chance to see what is actually
going on. Tested on the FVP fast model.

Acked-by: Steve Rae <srae@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>