depmod: pair in a modules.load.charger on BOARD_VENDOR_CHARGER_KERNEL_MODULES_LOAD

If the BoardConfig defined BOARD_VENDOR_KERNEL_MODULES_LOAD and
BOARD_VENDOR_CHARGER_KERNEL_MODULES_LOAD, generate both a
modules.load and modules.load.charger respectively on
/vendor/lib/modules.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 159424228
Bug: 151950334
Test: build, confirm, change to charger mode, confirm set of modules loads
Change-Id: I55677ed74d80ff3909fe8cbe26e6766f9b5283d5