Pass profile directory as parameter to mkarchiso

**archiso/mkarchiso**:

Change all override option parameters (i.e. `-A`, `-C`, `-D`, `-L`, `-P` and `-g`) to not directly override the global
variable they are tied to, but instead using an `override_` prefixed variable.

Add `_set_overrides()` to use `override_` prefixed variables (if set) to override those without a prefix.

Remove `-B` (a profile directory) from the list of parameters. The profile directory is now provided as separate
non-option parameter.

Add a call to `_read_profile()`, `_set_overrides()` and `command_build_profile()` to the fallthrough option of the
switch-case checking `command_name` - a non-option parameter to mkarchiso. This effectively provides the possibility to
set the profile directory using a non-option parameter, while still maintaining compatibility to legacy named arguments
used in the configs' `build.sh` scripts.

Extend the warning in regards to legacy `build.sh` based commands to mkarchiso by providing an EOL with archiso v49.

Change the help output to reflect the changes and further elaborate on the legacy commands used by `build.sh` scripts.

Change help output to be ordered alphabetically.

Add help output for `-r` and `-g` options.

Call `_set_overrides()` for legacy commands that accept one or more of the overriden options (i.e. `command_init`,
`command_install`, `command_prepare` and `command_iso`).

Various style fixes.

**configs/{baseline,releng}/build.sh**:

Change call to mkarchiso to use the profile's directory as a named argument instead of an option-argument.

**README.rst**:

Fix documentation on how to call mkarchiso with a profile directory.

Fix wording and ordering of option arguments for run_archiso documentation.

Fixes #52
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