Add linkermap target for size analysis

[hathach]

• linkermap target is added for size analysis
• ci also include linkermap output for quick and convenient check https://github.com/adafruit/Adafruit_nRF52_Bootloader/runs/3871902600?check_suite_focus=true#step:8:10 . Example output is

                                              File |   .text   .data    .bss   Total
------------------------------------------------------------------------------------
                                      msc_device.o |    1772       0    4160    5932
                                      ------------ |--------------------------------
                                  .bss._mscd_buf   |       0       0    4096    4096
                                    mscd_xfer_cb   |    1208       0       0    1208
               .text.proc_read10_cmd.constprop.0   |     172       0       0     172
                                       mscd_open   |     128       0       0     128
              .text.proc_write10_cmd.constprop.0   |     112       0       0     112
                            mscd_control_xfer_cb   |      96       0       0      96
                                  .bss._mscd_itf   |       0       0      64      64
                               tud_msc_set_sense   |      24       0       0      24
                                       mscd_init   |      16       0       0      16
                                      mscd_reset   |      16       0       0      16
------------------------------------------------------------------------------------
                                    hci_mem_pool.o |     348       0    4861    5209
                                    -------------- |--------------------------------
                     .bss.m_rx_buffer_elem_queue   |       0       0    4832    4832
                         hci_mem_pool_rx_consume   |     120       0       0     120
                         hci_mem_pool_rx_produce   |      84       0       0      84
                         hci_mem_pool_rx_extract   |      68       0       0      68
                               hci_mem_pool_open   |      40       0       0      40
                   hci_mem_pool_rx_data_size_set   |      32       0       0      32
                          .bss.m_rx_buffer_queue   |       0       0      28      28
                              hci_mem_pool_close   |       4       0       0       4
                          .bss.m_is_tx_allocated   |       0       0       1       1
------------------------------------------------------------------------------------
                                     flash_nrf5x.o |     140       4    4096    4240
                                     ------------- |--------------------------------
                                    .bss._fl_buf   |       0       0    4096    4096
                               flash_nrf5x_write   |      80       0       0      80
                               flash_nrf5x_flush   |      60       0       0      60
                                  .data._fl_addr   |       0       4       0       4
------------------------------------------------------------------------------------