Previously a value of 0 was used for the RSSI to signal that this value is not
present in `gnrc_netif_hdr_t`. However, an RSSI of 0 dBm is legal and even very
plausible data.
This commit defines `GNRC_NETIF_HDR_NO_RSSI` as `INT16_MIN`, which is below the
noise floor in the vacuum of outer space and hence impossible to receive.
For consistency, also GNRC_NETIF_HDR_NO_LQI is defined.
Reordered struct members to not waste memory due to padding.
Before:
``` C
typedef struct {
uint8_t src_l2addr_len;
uint8_t dst_l2addr_len;
kernel_pid_t if_pid; // <-- 16 bit, is aligned to 16 bit
uint8_t flags;
uint8_t __padding_byte; // <-- Inserted to fulfill padding requirements
int16_t rssi; // <-- 16 bit, is NOT aligned to 16 bit
uint8_t lqi;
uint8_t __padding_byte2;// <-- Inserted to fulfill padding requirements
} gnrc_netif_hdr_t;
```
Now:
``` C
typedef struct {
uint8_t src_l2addr_len;
uint8_t dst_l2addr_len;
kernel_pid_t if_pid; // <-- 16 bit, is aligned to 16 bit
uint8_t flags;
uint8_t lqi;
int16_t rssi; // <-- 16 bit, is aligned to 16 bit
} gnrc_netif_hdr_t;
```
When build for the `bluepill` board, the new layout reduces the size by 2 bytes.
