lib: Fix coldboot race condition observed on emulators/simulators

If we are running on RISC-V emulator/simulator with large number of
HARTs where each HART is a regular thread under UNIX host then it is
possible that some of the secondary HARTs don't get chance to run and
sbi_hart_wake_coldboot_harts() is called before secondary HARTs call
sbi_hart_wait_for_coldboot(). In this situation, some of the secondary
HARTs will never come-out of coldboot wait loop.

To tackle this, we introduce a global flag coldboot_done which will
be protected by coldboot lock and it will be set by primary HART from
sbi_hart_wake_coldboot_harts() before waking-up secondary HARTs. We
also re-arrange acquire/release of coldboot lock to reduce further
chances of race-condition.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Nylon <Chen&lt;nylon7@andestech.com>