fw_env - simply read a variable from U-Boot Env block

What it is

fw_env is a simple pure Rust implementation of fw_printenv from the U-Boot bootloader tools. Basic usage:

	let config = Config::init()?;
	let env = FwEnv::read(&config)?;
	let uboot_version = env.find_var(b"ver"[..]).expect("variable not found");

TODO

• check CRC
• find variable by name
• full config file (/etc/fw_env.config) parsing
• handle bad blocks in flash
• handle flags in redundant blocks