sinit: Add /bin/init symlink

This way, the kernel will find it by default without specifying
init=/bin/sinit or CONFIG_DEFAULT_INIT=/bin/sinit.
file: Use tool built for host to generate magic.mgc

It turns out file has an undocumented define COMPILE_ONLY that
enables building a tool to just generate the magic database with a
minimal set of sources. Use that to build a tool for the host system
to avoid version incompatibilities.

Fixes #24.
texi2mdoc from host system is no longer required

Since b93e7da0 we use a locally-built texi2mdoc for the host system.
adobe-source-fonts: Update for renamed source-sans repository
dav1d: Fix name of dependency target
adobe-source-fonts: Update to latest versions
adobe-source-fonts: Update source-serif-pro checksum

The repository was renamed to source-serif, and since we use the
autogenerated .tar.gz files instead of the static .zip, it caused
the paths in the archive to change respectively.
squashfs-tools-ng: Update to 1.0.4
e2fsprogs: Update to 1.45.7
binutils: Update to 2.36
u-boot: Update to 2021.01
youtube-dl: Update to 2021.01.24.1
nsd: Update to 4.3.5
pounce: Update to 2.2
catgirl: Update to 1.5
wayland: Update to 1.19.0
byacc, cacert: Add missed version bumps
tz: Update to 2021a
sfeed_curses: Update to 0.9.8
sfeed: Update to 0.9.21
pigz: Update to 2.5