Rework main Makefiles

Rework main Makefiles

This includes:

1. Finally, move SOC definitions to an external header file
   and just include it in config.h

2. Unify target naming scheme and use same names in both
   Makefiles, fix names for some targets, include device
   vendor name

3. Use custom functions to reduce code size and duplicates

4. IMPORTANT: Use LZMA-compressed image version by default,
   as the difference in boot time is negligible, but size
   of the code is reduced by more than 50%

5. Show some information about image configuration during
   building, example:

   Removing configuration...
   Preparing configuration for target: tp-link_tl-wdr3600_tl-43x0

     Device vendor/model:    TP-Link TL-WDR3600/43x0
     Custom recovery web:    no
     Image size limit:       123 KB
     Default FLASH size:     8 MB
     GPIO reset button:      16
     Button active low:      yes
     LZMA compression:       yes
     RAM-loadable only:      no

   [...]

   Copying compiled image...
   Checking size of the image...
   Preparing 123 KB image padded with 0xFF...
   Preparing final image...
   Calculating MD5 sum for the final image...

   DONE!
   Image 'bin/u-boot_mod__20160826__tp-link_tl-wdr3600_tl-43x0.bin' is ready!

6. Change final image name, include build date, proper name
   of the project (u-boot_mod) and selected target name