kmem.py: account for kernel data structures

Account for seL4 kernel objects. Detailed accounting shows kernel
objects using the mangled capDL names (modulo some stripping of
component prefixes to make display more compact).

Initially this accounting a riscv32 target architecture with MCS enabled.

While here fix assumption data structure sizes are at leat 4KB
(was true when only page frames were being tracked).

Add --verbose option to enable objects skipped for various reasons
(e.g. IRQ's that have no associated memory).

Add --kernel option to enable kernel data structure analysis.

Change-Id: Iddab2d9a71af3e11de00fa67c582212dfe9f7e7b