Add fexecwstr() a variant of fexecw()

fexecwstr() do the some thing of fxecw() but the stdout and stderr from
child are get and copy to buffer.

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Support for kexec's --dtb option implemented.

You can use DTB option in boot.cfg to specify a devicetree.

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Implement exec append command-line

User could set EXEC_APPEND in boot.cfg to execute a program.

The program must return a string and exit. Then kexecboot will
parse the outputed string looking for 'append='. If found it will
append to command-line that is to be passed to kexec.

For now, kexecboot can get 'append' and 'dtb' parameters from outputed string.
'dtb=' is passed to kexec.

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Reuse set_dtb function to get DTB from config file

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>