This lets us write modules which don't know the details of the RTOS
objects they use. More importantly, this lets us target those modules
at either Linux (using the pthread-based implementation in chip_stub)
or actual hardware, so we can write Linux-based unit tests.
BUG=None
TEST=make && ./ec_os_test
Change-Id: I2923b9a48248b27f3bc3b0346d5a870fef8a1c5b
(cherry picked from commit 6ccc37af9fb916f7ad70f542905ba8bacb8e3be2)