uboot-mvebu: backport patch to fix compilation on non glibc system

uboot-mvebu: backport patch to fix compilation on non glibc system

This issue was reported by @paper42, who is using Void Linux with musl
to compile OpenWrt and its packages and found out it is not possible to
compile U-boot for Turris Omnia (neither any other).

It fixes following output:
```
  HOSTCC  tools/kwboot
tools/kwboot.c: In function 'kwboot_tty_change_baudrate':
tools/kwboot.c:662:6: error: 'struct termios' has no member named 'c_ospeed'
  662 |   tio.c_ospeed = tio.c_ispeed = baudrate;
      |      ^
tools/kwboot.c:662:21: error: 'struct termios' has no member named 'c_ispeed'
  662 |   tio.c_ospeed = tio.c_ispeed = baudrate;
      |                     ^
tools/kwboot.c:690:31: error: 'struct termios' has no member named 'c_ospeed'
  690 |  if (!_is_within_tolerance(tio.c_ospeed, baudrate, 3))
      |                               ^
tools/kwboot.c:693:31: error: 'struct termios' has no member named 'c_ispeed'
  693 |  if (!_is_within_tolerance(tio.c_ispeed, baudrate, 3))
      |
```

Tested-by: Michal Vasilek <michal.vasilek@nic.cz>
Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>