Add a way to execute a large set of privileged commands on machine initialization

[edubart]

Context

Some command line options need to execute custom commands as root on boot, such as mounting a flash drive, initializing clock date and time, and initializing network. This feature is needed in the context of Hypervisor to boot hypervisor VMs, also in the context of VirtIO to properly initialize its drivers.

There is also a second problem about the limit of characters in the dapp command line that we must solve, because currently we cannot have a command line with more than ~4096 characters.

Possible solutions

We can either use an extra flash drive for supplying the commands or build them directly in the FDT.

Definitions of Done

• Add new command line option to execute large sequence of commands (or shell script) as root user
• Add new command line option to execute large sequence of commands (or shell script) as dapp user
• Make sure command line option can work with large amount of characters
• Simplify init script from tools, move some of its init to emulator, to make them optional
• Add options to suppress some initialization features currently always done in init, such as mounting flash drives