[kernel, build] Dynamically configure kernel when /bin/init not present

[ghaerr]

The kernel startup for task 1 was enhanced to allow for dynamically handling the case when the first task is not /bin/init.
Previously, this required special kernel compilation using CONFIG_SYS_NO_BININIT. Now, the kernel handles this dynamically, which can occur when /bootopts uses init= or /bin/init is missing from the boot image. In addition, the meaning of CONFIG_SYS_NO_BININIT is changed when set in a config file to not copy /bin/init, /bin/getty and /bin/login, resulting in smaller systems, mostly useful for ROM systems.

Discussed in more detail in #2426. This PR also includes several enhancements originally written or conceived by @hexadec1mal in #2426. In particular, @hexadec1mal's ideas or code is used:

• Using CONFIG_SYS_NO_BININIT for controlling image contents
• Adding :init tag in Applications installation file
• Better language in menuconfig when CONFIG_SYS_NO_BININIT used
• Fixes in .gitignore and parallel makes

Thank you @hexadec1mal!

The previous mechanism for disabling auto-reaping of child processes on parent exit when /bin/init is not running was broken. sash now calls waitpid to remove zombie processes, which caused an out-of-tasks condition when /bin/sash was run as the init task.

Tested on PC builds and emu86 ROM images. Should work on all other images since CONFIG_SYS_NO_BININIT is not set by default for any supported port.