arch/arm: fix-up the ARM Kconfig warning

Kconfig does not accepts that a symbol that is part of a choice
be affected a default value.

Fix this by introducing a dummy EABI symbol, and make the real
EABI symbol a prompt-less option that depends on !OABI.

[Peter: drop arm dependency, rename to EABI_CHOICE]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>