This page:

  https://www.nongnu.org/avr-libc/user-manual/pgmspace.html

contains this:

  #include <avr/pgmspace.h>
.
.
.
unsigned char mydata[11][10] PROGMEM =
{
    {0x00,0x01,0x02,0x03,0x04,0x05,0x06,0x07,0x08,0x09},
    {0x0A,0x0B,0x0C,0x0D,0x0E,0x0F,0x10,0x11,0x12,0x13},
    {0x14,0x15,0x16,0x17,0x18,0x19,0x1A,0x1B,0x1C,0x1D},
    {0x1E,0x1F,0x20,0x21,0x22,0x23,0x24,0x25,0x26,0x27},
    {0x28,0x29,0x2A,0x2B,0x2C,0x2D,0x2E,0x2F,0x30,0x31},
    {0x32,0x33,0x34,0x35,0x36,0x37,0x38,0x39,0x3A,0x3B},
    {0x3C,0x3D,0x3E,0x3F,0x40,0x41,0x42,0x43,0x44,0x45},
    {0x46,0x47,0x48,0x49,0x4A,0x4B,0x4C,0x4D,0x4E,0x4F},
    {0x50,0x51,0x52,0x53,0x54,0x55,0x56,0x57,0x58,0x59},
    {0x5A,0x5B,0x5C,0x5D,0x5E,0x5F,0x60,0x61,0x62,0x63},
    {0x64,0x65,0x66,0x67,0x68,0x69,0x6A,0x6B,0x6C,0x6D}
};

With warning options, on avr-gcc (version 7.3.0) this results in this compile error:

main_bootloader.c:28:15: error: variable 'mydata' must be const in order to be put into read-only section by means of '__attribute__((progmem))'

Changing the first line to begin 'unsigned char const' makes the build work (as the error suggests it would).  Since the definition is const it would probably be best to include the const keyword.