nanobsd: Avoid read-only file system warnings

The default setting of some loader or rc variables generate warnings
because the file system is read-only:

Set the following options to avoid printing these warnings:

/boot/defaults/loader.conf:

• hostuuid_load="NO" to disable loading /etc/hostid early at boot time
• entropy_cache_load="NO" to disable loading cached entropy at boot time
• kern.random.initial_seeding.disable_bypass_warnings="1" to disable logging a warning if the 'bypass_before_seeding' knob is enabled (enabled by default) and a request is submitted prior to initial seeding

/etc/defaults/rc.conf:

• kldxref_enable="NO" to disable building linker.hints files with kldxref(8)
• entropy_boot_file="NO" to disable very early (used at early boot-time) entropy caching through reboots
• entropy_dir="NO" to disable caching entropy via cron

While here, move rc.conf options before sourcing vendor.conf, so they
can be overridden.

PR: 277601
Reviewed by: imp
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47502

(cherry picked from commit e5579f98c02ff64b0c305f6731c3557ba69c2d25)