build: order the mount tools before bootfs for fs=ramfs

[gburd]

Building a fresh tree with fs=ramfs fails during MKBOOTFS:

strip: 'tools/mount/mount-fs.so': No such file
FileNotFoundError: [Errno 2] No such file or directory: 'tools/mount/mount-fs.so'
make: *** [Makefile:2238: build/release.x64/bootfs.bin] Error 1

Cause

For fs=ramfs, scripts/build uses the generated $(out)/usr.manifest as the bootfs manifest, and that manifest is generated from usr_ramfs.manifest.skel, which names two tools:

/tools/mount-fs.so: tools/mount/mount-fs.so
/tools/umount.so: tools/mount/umount.so

bootfs_dep, however, only gains the tools for fs=ext and fs=zfs:

bootfs_dep := scripts/mkbootfs.py $(bootfs_manifest) $(bootfs_manifest_dep) $(out)/libenviron.so
ifeq ($(fs),ext)
bootfs_dep += $(out)/modules/libext/libext.so
else
ifeq ($(fs),zfs)
bootfs_dep += $(tools:%=$(out)/%) $(out)/libsolaris.so
endif
endif

So for fs=ramfs nothing orders those two objects before bootfs.bin, and mkbootfs.py stats a file make was never told to build first.

It is timing dependent, which is why it is not always visible: in a tree that already built something else the tools may happen to be present. In a clean clone it fails every time, because bootfs.bin is reached before anything else has a reason to build the mount tools.

Fix

Add just those two objects to bootfs_dep for fs=ramfs. Only the files the ramfs manifest actually names are added, rather than all of $(tools), so a ramfs build does not start pulling in mkfs/cpiod/uush, which it never references.

Verification

I checked this against a clean clone of this repository's master rather than only against my own tree, since that is where it reproduces most clearly:

build	result
clean clone, fs=ramfs, without this change	fails at MKBOOTFS (FileNotFoundError)
clean clone, fs=ramfs, with this change	completes, image produced

I also re-ran the default (zfs) and fs=rofs builds with the change applied and both still complete, so the added dependency does not disturb the other filesystem paths.

Note this is a distinct problem from the bootfs ZFS-library dependency in #1494: that one is about libsolaris.so and the ZFS tools for a ZFS-enabled build, while this is the ramfs manifest's own two mount tools going unordered.