Configuring of MKS_GEN_L V1.0 I've bought a new "MKS GEN_L V1.0" and I'm trying to configure it with Marlin 1.1.X. I changed motherboard in configuration.h from previous:

#define BOARD_RAMPS_13_EFB   33 //RAMPS 1.3 (Power outputs:Hotend,Fan,Bed) 

to 

#define BOARD_MKS_GEN_L  53 //MKS GEN L. 

I'm getting this error:

pins.h:268: error: #error "**Unknown MOTHERBOARD value set in Configuration.h**"
       #error "Unknown MOTHERBOARD value set in Configuration.h"
        ^
    In file included from sketch\MarlinConfig.h:42:0,
                     from sketch\G26_Mesh_Validation_Tool.cpp:27:

SanityCheck.h:58: error: #error "MOTHERBOARD is required. Please update your configuration."
       #error "MOTHERBOARD is required. Please update your configuration."
        ^
    SanityCheck.h:786: error: #error "**Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN requires the Z_MIN_PIN to be defined.**"
           #error "Z_MIN_PROBE_USES_Z_MIN_ENDSTOP_PIN requires the Z_MIN_PIN to be defined."

        ^
    SanityCheck.h:942: error: #error "**LCD_BED_LEVELING requires an LCD controller.**"
         #error "LCD_BED_LEVELING requires an LCD controller."
          ^

SanityCheck.h:1084: error: #error "**HEATER_0_PIN not defined for this board.**"
       #error "HEATER_0_PIN not defined for this board."
        ^
    exit status 1
#error "Unknown MOTHERBOARD value set in Configuration.h"
The motherboard definition in configuration.h should be written like:

#define MOTHERBOARD BOARD_MKS_GEN_L


actually the word MOTHERBOARD was missing and there is no need to write  53 //MKS GEN L at the end.

The number definition is declared in boards.h - you just confused the two files, indeed you should not forget to define the motherboard constant itself. Do note that this is basically a RAMPS board, see pins_MKS_GEN_L.h.