init: cleanup is_first_stage conditionals

A recent change to the is_first_stage conditionals created a unneeded
else { } block as both the code in the else { } block and any code
that runs after it are both in the second stage of init.  A first step
to clean this up is to remove this else block.

Secondly, given the above confusion, it makes sense to simplify the two
if (is_first_stage) conditions into one, which only now requires
duplicating one line to initialize logging and the actual "init
first/second stage started!" logs.

Lastly, there are a few commands ran at the beginning of both init
stages that do not need to be,

* boot_clock::time_point start_time = boot_clock::now();
This is only used in the first stage so keep it there

* umask(0);
umasks are preserved across execve() so it only needs to be set in the
first stage

* chmod("/proc/cmdline", 0440);
This needs to be moved until after /proc is mounted in the first
stage, but otherwise only needs to be done once

Test: Boot bullhead, check umask, check cmdline permissions, check
boot time property

Change-Id: Idb7df1d4330960ce282d9609f5c62281ee2638b9