arc: Update variables substitutions in u-boot env files

arc: Update variables substitutions in u-boot env files

In the latest version of u-boot (2018.05) there was a swith to
Hush shell for ARC AXS10x boards(arc770/archs38):
commit 9249d74781e1 ("ARC: AXS10x: Enable hush shell").
In Hush shell using "$()" to declare envitonment variables is forbidden,
instead of this "${}" need to be used.

Signed-off-by: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: John Crispin <john@phrozen.org>