arm: make 'MAKEALL -a' distinguish between arm and aarch64

The vexpress_aemv8a is the first aarch64 board in U-Boot.
As it was introduced, it gets built when "MAKEALL -a arm"
is invoked, and fails as this command is run with a 32-bit,
not 64-bit, toolchain as the cross-compiler.

Introduce 'aarch64' as a valid 'MAKEALL -a' argument, treated
as 'arm' for all other intents, and change the architecture
of the vexpress_aemv8a entry in boards.cfg from 'arm' to
'aarch64'.
diff --git a/boards.cfg b/boards.cfg
index e168590..029553d 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -397,7 +397,7 @@
 Active  arm         pxa            -           icpdas          lp8x4x              lp8x4x                               -                                                                                                                                 Sergey Yanovich <ynvich@gmail.com>
 Active  arm         pxa            -           toradex         -                   colibri_pxa270                       -                                                                                                                                 Marek Vasut <marek.vasut@gmail.com>
 Active  arm         sa1100         -           -               -                   jornada                              -                                                                                                                                 Kristoffer Ericson <kristoffer.ericson@gmail.com>
-Active  arm         armv8          -           armltd          vexpress64          vexpress_aemv8a                      vexpress_aemv8a:ARM64                                                                                                             David Feng <fenghua@phytium.com.cn>
+Active  aarch64     armv8          -           armltd          vexpress64          vexpress_aemv8a                      vexpress_aemv8a:ARM64                                                                                                             David Feng <fenghua@phytium.com.cn>
 Active  avr32       at32ap         at32ap700x  atmel           -                   atngw100                             -                                                                                                                                 Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
 Active  avr32       at32ap         at32ap700x  atmel           -                   atngw100mkii                         -                                                                                                                                 Andreas Bießmann <andreas.devel@googlemail.com>
 Active  avr32       at32ap         at32ap700x  atmel           atstk1000           atstk1002                            -                                                                                                                                 Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
@@ -1242,4 +1242,3 @@
 Orphan  powerpc     mpc8xx         -           -               mbx8xx              MBX                                  -                                                                                                                                 -
 Orphan  powerpc     mpc8xx         -           -               mbx8xx              MBX860T                              -                                                                                                                                 -
 Orphan  powerpc     mpc8xx         -           -               nx823               NX823                                -                                                                                                                                 -
-