openocd: strip leading '0x' from hex numbers returned by openocd

[cbrune]

This patch strips leading '0x' from hex numbers (if present) returned by openocd for commands like mem2array and mrw.

Fixes: #560

With this patch, the issue described by #560 does not happen:

$ cargo xtask humility app/demo-stm32h7-nucleo/app-h753.toml -- tasks
    Finished `dev` profile [optimized + debuginfo] target(s) in 0.18s
     Running `target/debug/xtask humility app/demo-stm32h7-nucleo/app-h753.toml -- tasks`
humility: attached via OpenOCD
system time = 6695667
ID TASK                       GEN PRI STATE    
 0 jefe                         0   0 recv, notif: fault timer(T+33)
 1 sys                          0   1 recv, notif: exti-wildcard-irq(irq6/irq7/irq8/irq9/irq10/irq23/irq40)
 2 i2c_driver                   0   2 recv
 3 packrat                      0   2 recv
 4 spi_driver                   0   2 recv
 5 net                          0   2 recv, notif: eth-irq(irq61) wake-timer
 6 user_leds                    0   2 recv, notif: timer(T+334)
 7 user_button                  0   3 notif: button
 8 udpecho                      0   3 notif: socket
 9 udpbroadcast                 0   3 notif: bit31(T+1026)
10 udprpc                       0   3 notif: socket
11 hiffy                        0   5 notif: bit31(T+512)
12 hf                           0   4 notif: bit31(T+1037)
13 hash_driver                  0   3 recv
14 idle                         0   7 RUNNING
15 rng_driver                   0   3 recv
16 dump_agent                   0   4 recv

[cbrune]

lol, this is trying to solve the same issue as PR #500, which I didn't see before now.