manifesto: add mnfb parser

[cfriedt]

Previously the MnfbParser class was just a stub, so e.g. the following
would fail:

manifesto -I mnfb -O mnfs input.mnfb
Error: 'MnfbParser' object has no attribute 'parse_file'

This change adds a very simple binary file parser.

example output:

Warning: non-incremental id for '[None]'
Warning: non-incremental id for '[None]'
[manifest-header]
version-major = 0
version-minor = 1

[interface-descriptor]
vendor-string-id = 0x1
vendor-product-id = 0x2

; Interface vendor string
[string-descriptor 0x1]
string = Zephyr Project RTOS

; Interface product string
[string-descriptor 0x2]
string = Greybus Service Sample Application

; 'Control' class on Bundle 0
[bundle-descriptor 0x0]
class = 0x0

; 'Bridged PHY' class on Bundle 1
[bundle-descriptor 0x1]
class = 0xa

; 'Bridged PHY' class on Bundle 2
[bundle-descriptor 0x2]
class = 0xa

; 'Bridged PHY' class on Bundle 3
[bundle-descriptor 0x3]
class = 0xa

; 'Control' protocol on CPort 0
[cport-descriptor 0x0]
bundle = 0x0
protocol = 0x0

; 'GPIO' protocol on CPort 1
[cport-descriptor 0x1]
bundle = 0x1
protocol = 0x2

; 'I2C' protocol on CPort 2
[cport-descriptor 0x2]
bundle = 0x2
protocol = 0x3

; 'SPI' protocol on CPort 3
[cport-descriptor 0x3]
bundle = 0x3
protocol = 0xb