Add threads support

milestone4
Malloc space for threads exception handler

milestone4
Dissable UART cap

milestone4
Retype IO cap into UART

mileston4
Add interrupts for UART RX
Refactor init

milestone4
Add alignment for heap head

milestone4
Add check if frame bytes < BASE_PAGE_SIZE in paging_map_frame_attr

milestone4
Use relative address for UART registers

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Add aos_rpc debug macros

milestone4
Use Mackerel for accessing UART registers

milestone4
Handle UART interrupts in user space

Conflicts:
	usr/init/init.c
	usr/init/init.h

milestone4
Add dummy get_char handler

milestone4
Add dummy get_char call from memeater
Start implementing shell
Merge remote-tracking branch 'origin/djole_m4_new' into milestone/4

Conflicts:
	usr/init/init.c
	usr/memeater/memeater.c
Glue aos_rpc_serial_* to libc

milestone4
Add shell with echo!

milestone4
Merge remote-tracking branch 'origin/djole_m4_new' into milestone/4

Conflicts:
	usr/init/init.c
	usr/memeater/memeater.c
Add memtester

milestone4