oeqa/postactions: Do not use -l option with df

-l option is specific to df provided by coreutils, if df
applet from busybox is used then it does not work and fails
like below

Fixes
   df: invalid option -- 'l'
   BusyBox v1.36.1 () multi-call binary.

   Usage: df [-PkmhT] [-t TYPE] [FILESYSTEM]...
   DEBUG: [Command returned '1' after 0.71 seconds]
   DEBUG: Command: df -hl
   Status: 1 Output:  df: invalid option -- 'l'

it seems worth a compromise to show remote mounted filesystems if any
during ptests and it works with both df implementations

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>