chore: fix constness to avoid having constants in data segment

[raphaelcoeffic]

.... and thus free around 3KB precious RAM.

Requires EdgeTX/libopenui#84

X7

*** X7 before ***

Section Details:
section              size(B)         addr   loc
.text                 466272   0x08000000   Flash
.ARM                       8   0x08071d60   Flash
.init_array               52   0x08071d68   Flash
.fini_array                4   0x08071d9c   Flash
.data (added)           3544   0x08071da0   Flash
.data                   3544   0x20000000   SRAM1
.bss                   52356   0x20000dd8   SRAM1
._user_heap_stack       1024   0x2000da5c   SRAM1

Totals         KB    usage  of ttl	KB free
  Flash:   458.87   89.62%     512	  53.13
  SRAM1:    55.59   49.63%     112	  56.41
  SRAM2:     0.00    0.00%      16	  16.00


*** X7 after ***

Section Details:
section              size(B)         addr   loc
.text                 469388   0x08000000   Flash
.ARM                       8   0x0807298c   Flash
.init_array               52   0x08072994   Flash
.fini_array                4   0x080729c8   Flash
.data (added)            428   0x080729cc   Flash
.data                    428   0x20000000   SRAM1
.bss                   52356   0x200001b0   SRAM1
._user_heap_stack       1024   0x2000ce34   SRAM1

Totals         KB    usage  of ttl	KB free
  Flash:   458.87   89.62%     512	  53.13
  SRAM1:    52.55   46.92%     112	  59.45
  SRAM2:     0.00    0.00%      16	  16.00

==> 3116 bytes free SRAM more!

TPRO

*** TPRO before ***

Section Details:
section              size(B)         addr   loc
.text                 491112   0x08000000   Flash
.ARM                       8   0x08077e68   Flash
.init_array               48   0x08077e70   Flash
.fini_array                4   0x08077ea0   Flash
.data (added)           3664   0x08077ea4   Flash
.data                   3664   0x20000000   SRAM1
.bss                   57356   0x20000e50   SRAM1
._user_heap_stack       1024   0x2000ee5c   SRAM1

Totals         KB    usage  of ttl	KB free
  Flash:   483.24   94.38%     512	  28.76
  SRAM1:    60.59   54.10%     112	  51.41
  SRAM2:     0.00    0.00%      16	  16.00

*** TPRO after ***

Section Details:
section              size(B)         addr   loc
.text                 494316   0x08000000   Flash
.ARM                       8   0x08078aec   Flash
.init_array               48   0x08078af4   Flash
.fini_array                4   0x08078b24   Flash
.data (added)            460   0x08078b28   Flash
.data                    460   0x20000000   SRAM1
.bss                   57356   0x200001d0   SRAM1
._user_heap_stack       1024   0x2000e1dc   SRAM1

Totals         KB    usage  of ttl	KB free
  Flash:   483.24   94.38%     512	  28.76
  SRAM1:    57.46   51.30%     112	  54.54
  SRAM2:     0.00    0.00%      16	  16.00

==> 3204 bytes free SRAM more!

[pfeerick]

It seems like double that on the radio itself... compared with PR 2990, with this PR the X9D+ went from 60836 free mem to 67036 (per the statistics/debug page), and T-Pro went from 67828 to 71024 😮 Negligible effect on the TX16S from 5307736 became 5307564 - a reduction of 172 bytes 😆 - but who cares with that much of the 8Mb SDRAM still free (as well as being understandable as most of the changes were in B&W UI code).