arm: fix compiler warnings

There were unused variables, incorrect pointer operations
and plan broken string comparison.

Now there's only one - for unused uart3_enabled in beaglebone.c,
but we want to keep it for declaration consistency.

Also fixes #757.

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>

src/arm/phyboard.c

@@ -98,7 +98,6 @@ mraa_pwm_context
 mraa_phyboard_pwm_init_replace(int pin)
 {
     char devpath[MAX_SIZE];
-    int length = strlen(SYSFS_CLASS_PWM) + 5;
 
     if (plat == NULL) {
         syslog(LOG_ERR, "pwm: Platform Not Initialised");