add firmware format documentation

docs/bootloader.md

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+#TREZOR OS Bootloader
+
+##Firmware file format
+
+TREZOR OS firmware file consists of 3 parts:
+
+1. vendor header
+2. firmware header
+3. firmware code
+
+###Vendor Header
+
+| offset | length | name | description |
+|-------:|-------:|------|-------------|
+| 0x0000 | 4      | magic | firmware magic `TRZV` |
+| 0x0004 | 4      | hlen | length of the vendor header (big endian) |
+| 0x0008 | 1      | vsig_m | number of signatures needed to run the firmware from this vendor |
+| 0x0009 | 1      | vsig_n | number of pubkeys vendor wants to use for signing |
+| 0x000A | 1      | vstr_len | vendor string length |
+| 0x000B | ?      | vstr | vendor string |
+| ?      | ?      | vimg | vendor image (in [TOIf format](toif.md)) |
+| ?      | 32     | vpub1 | vendor pubkey 1 |
+| ...    | ...    | ... | ... |
+| ?      | 32     | vpubn | vendor pubkey n |
+| ?      | 64     | slsig1 | SatoshiLabs signature 1 |
+| ?      | 64     | slsig2 | SatoshiLabs signature 2 |
+| ?      | 64     | slsig3 | SatoshiLabs signature 3 |
+
+###Firmware Header
+
+| offset | length | name | description |
+|-------:|-------:|------|-------------|
+| 0x0000 | 4      | magic | firmware magic `TRZF` |
+| 0x0004 | 4      | hlen | length of the firmware header |
+| 0x0008 | 4      | codelen | length of the firmware code |
+| 0x000C | 1      | vmajor | version (major) |
+| 0x000D | 1      | vminor | version (minor) |
+| 0x000E | 1      | vpatch | version (patch) |
+| 0x000F | 1      | vbuild | version (build) |
+| 0x0010 | 1      | vidx1 | vendor signature index 1 |
+| 0x0011 | 32     | vsig1 | vendor signature 1 |
+| 0x0043 | 1      | vidx2 | vendor signature index 2 |
+| 0x0044 | 32     | vsig2 | vendor signature 2 |
+| ...    | ...    | ...   | ... |
+| ?      | 1      | vidxn | vendor signature index n |
+| ?      | 32     | vsign | vendor signature n |