Changes

firmware: replace libnewlib with picolibc

libnewlib-arm-none-eabi has been removed in debian unstable, build the
firmware with picolibc instead.

Fixes: OS#7017
Change-Id: If136397b71435ea0850002ef6406067962e8d4d7

firmware/Makefile: remove -Wredundant-decls

picolibc has redundant declarations of free and strdtold, which results
in lots of warnings with -Wredundant-decls. Adjust CFLAGS to remove
-Wredundant-decls to make the compiler log more readable.

  /usr/lib/picolibc/arm-none-eabi/include/stdlib.h:147:9: warning: redundant redeclaration of 'free' [-Wredundant-decls]
    147 | void    free (void *) __nothrow;
        |         ^~~~
  /usr/lib/picolibc/arm-none-eabi/include/stdlib.h:99:9: note: previous declaration of 'free' with type 'void(void *)'
     99 | void    free (void *) __nothrow;
        |         ^~~~
  /usr/lib/picolibc/arm-none-eabi/include/stdlib.h:307:9: warning: redundant redeclaration of 'strtold' [-Wredundant-decls]
    307 |         strtold (const char *__restrict, char **__restrict);
        |         ^~~~~~~
  /usr/lib/picolibc/arm-none-eabi/include/stdlib.h:286:9: note: previous declaration of 'strtold' with type 'long double(const char * restrict,  char ** restrict)'
    286 |         strtold (const char *__restrict __n, char **__restrict __end_PTR);
        |         ^~~~~~~

Change-Id: I32ff78f2a27191d1c6e114429d2e0713674732fe