buildroot

rpi-firmware: unbreak _INSTALL_VCDBG logic

Commit d16b32a401 (package/rpi-firmware: add option to install vcdbg) added
a sub option to install the vcdbg tool, but the commit unfortunately
contained two errors:

- The package sets _INSTALL_TARGET = NO, so the _TARGET_CMDS are never used
- The install command used -d (handle arguments as directories) instead of
  -D (create needed directories)

Fix these issues.

Cc: yann.morin.1998@free.fr
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>