ANDROID: Revert "arm64: Drop support for CMDLINE_EXTEND"

This reverts commit cae118b6.

Upstream removed the support for CMDLINE_EXTEND on arm64 due to some
inconsistencies in the way command-line was assembled together. However,
the patch inadvertently deleted same config option from the android tree
which was first introduced by commit 056f1281 ("ANDROID: arm64: copy
CONFIG_CMDLINE_EXTEND from ARM") back in 2014.

Upstream is currently working on a solution to fix the divergence by
allowing prepend and append functionalities. While we wait for the new
series to land let's bring back CMDLINE_EXTEND option for android, so
arm64 can actually use of the functionality provided by d9a2a3f2
("ANDROID: of: Support CONFIG_CMDLINE_EXTEND config option").

Bug: 120440972
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I051f445641c8fbc3b8a13015d948f553d87cd791