This commit changes only ones with the default 2048 and
leaves the others.
E.g. this leaves SYSTEM_RAMTEST_STACKSIZE, whose default is 1024.
I guess those need to be inspected one-by-one.
1. Check NSH_NETINIT for self network initialization
2. Check NSH_ARCHINIT for sel arch specific initialization
3. Always show help regardless of NSH_BUILTIN_APPS
4. Loop forever regardless of NSH_BUILTIN_APPS, user could:
a.change the default behavior by the command line argument
b.or ctrl+c to break out the loop
