newt/builder: read BOOTUTIL_MAX_IMG_SECTORS from syscfg for boot trailer

Boot trailer size was hardcoded to 128 sectors, ignoring the value
set in syscfg. This caused mismatches between trailer size and
the actual image size when BOOTUTIL_MAX_IMG_SECTORS was set to
value other than 128, potentially making images not fit
in the slot. Now the trailer size is calculated based on the syscfg
value.

Author: wiktorkwiatkowski

newt/builder/targetbuild.go

@@ -777,6 +777,7 @@ func (t *TargetBuilder) InjectSetting(key string, value string) {
 // that must be reserved at the end of each image slot.
 func (t *TargetBuilder) bootTrailerSize() int {
 	var minWriteSz int
+	var maxImgSectors int
 
 	entry, ok := t.res.Cfg.Settings["MCU_FLASH_MIN_WRITE_SIZE"]
 	if !ok {
@@ -797,6 +798,26 @@ func (t *TargetBuilder) bootTrailerSize() int {
 		}
 	}
 
+	sectors, ok := t.res.Cfg.Settings["BOOTUTIL_MAX_IMG_SECTORS"]
+	if !ok {
+		util.StatusMessage(util.VERBOSITY_DEFAULT,
+			"* Warning: target does not define BOOTUTIL_MAX_IMG_SECTORS "+
+				"setting; assuming a value of 128.\n")
+		maxImgSectors = 128
+	} else {
+		val, err := util.AtoiNoOct(sectors.Value)
+		if err != nil {
+			util.StatusMessage(util.VERBOSITY_DEFAULT,
+				"* Warning: target specifies invalid non-integer "+
+					"BOOTUTIL_MAX_IMG_SECTORS setting; assuming a "+
+					"value of 128.\n")
+			maxImgSectors = 128
+		} else {
+			maxImgSectors = val
+		}
+	}
+
+
 	/* Mynewt boot trailer format:
 	 *
 	 *  0                   1                   2                   3
@@ -814,12 +835,13 @@ func (t *TargetBuilder) bootTrailerSize() int {
 	 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 	 */
 
+
 	tsize := 16 + // Magic.
-		128*minWriteSz*3 + // Swap status.
+		maxImgSectors*minWriteSz*3 + // Swap status.
 		minWriteSz + // Copy done.
 		minWriteSz // Image Ok.
 
-	log.Debugf("Min-write-size=%d; boot-trailer-size=%d", minWriteSz, tsize)
+	log.Debugf("Min-write-size=%d; max-img-sectors=%d; boot-trailer-size=%d", minWriteSz,  maxImgSectors, tsize)
 
 	return tsize
 }