arch: merge Config.in.aarch64 into Config.in.arm

arch: merge Config.in.aarch64 into Config.in.arm

The 64 bits ARM processors are capable of running 32 bits ARM code, and
some platforms are indeed using this capability. Due to this, if we were
to keep the separation between Config.in.aarch64 and Config.in.arm, we
would have to duplicate the definition of all 64-bits capable ARM cores
into both files.

Instead of going down this route, let's take the same route as the x86
one: a single Config.in.x86 file, used for both x86 32 bits and x86 64
bits, with the appropriate logic to only show the relevant cores
depending on which architecture is selected.

In order to do this, we:

 - Make the "ARM instruction set" choice only visible on ARM 32 bits,
   since we currently don't support ARM vs. Thumb on AArch64.

 - Add the relevant values for the BR2_ARCH option.

 - Add the relevant values for the BR2_ENDIAN option.

 - Make the "aapcs-linux" BR2_GCC_TARGET_ABI value only used on ARM 32
   bits, since this ABI doesn't mean anything on AArch64.

 - Make the BR2_GCC_TARGET_FPU option depends on ARM 32 bits, since
   there is no -mfpu option on AArch64.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>