Rename mainline_system(.*).mk to generic_system(.*).mk

Rename mainline_system to generic_system in mk files
to avoid confusion with mainline module.

First, move mainline_system(.*).mk to generic_system(.*).mk
and make previous mainline_system(.*).mk to just inherit
generic_system(.*).mk to make alias except mainline_system.mk.
is just symlink to generic_system.mk.

These aliasing file will be kept remain during migration
for preventing build breakage and making changes possible
during migration.

Bug: 160649682
Test: lunch generic_system_arm64 && m nothing
Test: lunch generic_system_x86 && m nothing
Test: lunch generic_system_x86_64 && m nothing
Test: lunch generic_system_x86_arm && m nothing
Test: lunch mainline_system_arm64 && m nothing
Test: lunch mainline_system_x86 && m nothing
Test: lunch mainline_system_x86_64 && m nothing
Test: lunch mainline_system_x86_arm && m nothing
Change-Id: I3609ec2d91b0f08859a36cac75c25876b1db6d6a
Merged-In: I3609ec2d91b0f08859a36cac75c25876b1db6d6a
(cherry picked from commit 6f16b576)