[source] bcm53xx: use iflag=skip_bytes for dd command during sysupgrade

[LEDE Commits]

rmilecki pushed a commit to source.git, branch master:
https://git.lede-project.org/3267ce750e491ba0290ec3a9a5e3b4dceadd47a5

commit 3267ce750e491ba0290ec3a9a5e3b4dceadd47a5
Author: Rafał Miłecki <rafal at milecki.pl>
AuthorDate: Sat Dec 23 23:14:25 2017 +0100

    bcm53xx: use iflag=skip_bytes for dd command during sysupgrade
    
    Since BusyBox 1.25.0 dd command supports iflag=skip_bytes which allows
    skipping requested amount of bytes without reducing blocksize. Thanks to
    this we can leave default blocksize and let dd work more efficiently.
    
    On Netgear R6250 "dd skip=58 iflag=skip_bytes" can be 5 times faster
    than "dd bs=58 skip=1" when extracting TRX out of CHK.
    
    Signed-off-by: Rafał Miłecki <rafal at milecki.pl>
---
 target/linux/bcm53xx/base-files/lib/upgrade/platform.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh b/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh
index 1a33e3a..06451f1 100644
--- a/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh
+++ b/target/linux/bcm53xx/base-files/lib/upgrade/platform.sh
@@ -274,11 +274,11 @@ platform_pre_upgrade() {
 platform_trx_from_chk_cmd() {
 	local header_len=$((0x$(get_magic_long_at "$1" 4)))
 
-	echo -n dd bs=$header_len skip=1
+	echo -n dd skip=$header_len iflag=skip_bytes
 }
 
 platform_trx_from_cybertan_cmd() {
-	echo -n dd bs=32 skip=1
+	echo -n dd skip=32 iflag=skip_bytes
 }
 
 platform_img_from_safeloader() {