New RISCV board (cartesi-machine)

[Marcelo Politzer]

Hi, I'm new to barebox

I'm currently porting barebox to a riscv board (cartesi-machine).

I got it mostly working:
- new serial: serial_sbi.c and
- ext4 filesystem with mtdram

1) What I am still trying to figure out is how to customize `env` correctly.
I got it to build and appear based on the docs [1], [2],
however the only way I found around [3] was to replace `bin/init` with
this[4] layout.
Is there a recommended way to go about this?

2) I also would like clarification on the upstreaming process, is it
PR based, patches on this list?

best,
Marcelo


[1] Makefile:
```
bbenv-$(CONFIG_BOARD_CARTESI) += defaultenv-cartesi
```

[2] arch/riscv/boards/cartesi:
```
defaultenv-cartesi
├── boot
│   └── cartesi
├── init
│   ├── automount
│   └── ps1
└── nv
    ├── allow_color
    ├── autoboot_timeout
    └── user
```

[3]
```
Hit m for menu or any to stop autoboot:    0
Booting entry 'net'
```

[4]
```
defaultenv-cartesi
├── bin
│   └── init
└── boot
    └── cartesi
```